Posts tagged with "node"

Level Up Your JavaScript Game! - Other ES6 Language Features

See Level Up Your JavaScript Game! for related content.

Sometimes it takes a while to learn new language features, because many are semantic improvements that aren’t absolutely necessary to get work done. Learning new features right away though is a great way to get ahead. Putting off learning new features leaves you lagging the crowd and constantly feeling like you’re catching up. I’ve noticed that junior developers often know more modern language features than senior developers.

There are quite a few language features that were introduced in ES5 and ES6, and you’d be well off to learn them all! Certainly, though, look into at least the ones I’m going to talk about here. I recommend you learn…

…to effectively use the object and array spread operators.

From MDN: “Spread syntax allows an iterable such as an array expression or string to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) are expected, or an object expression to be expanded in places where zero or more key-value pairs (for object literals) are expected.”

The spread operator is an ellipsis (...), but don’t confuse it with the pre-existing rest operator (also an ellipsis). The rest operator is used in the argument list of a function definition. The spread operator on the other hand is used… well, I’ll show you.

Think of the spread operator’s function as breaking the elements of an array (or the properties of an object) out into a comma delimited list. So [1,2,3] becomes 1,2,3. The array spread operator is most helpful for either passing elements to a function call as arguments or constructing a new array. The object spread operator is most helpful for constructing or merging objects properties.

If you have an array of values, you can pass them to a function call as separate arguments like this…

myFunction(...[1,2,3]);
//equivalent to myFunction(1,2,3)

If you have two objects - A and B - and you want C to be a superset of the properties on A and B you do this…

let C = {...A, ...B};

Therefore…

{...{"name":"Sally"},...{"age":10}}
//{"name":"Sally", "age":10}

…to get into the habit of using destructuring where appropriate.

Destructuring looks like magic when you first see it. It’s not just a gimmick, though. It’s quite useful.

Destructuring allows you to assign variables (on the left hand side of the assignment operator (=)) using an object or array pattern. The assignment will use the pattern you provide to extract values out of an object or array and put them where you want them.

let {name,age} = {name:"Sally",age:10};
//name == "Sally"
//age == 10

That’s a lot better than the alternative…

let person = {name:"Sally",age:10};
let name = person.name;
let age = person.age;

It works with nested properties too…

let {name,address.zip:zip} = {name:"Sally",age:10,address:{city:"Seattle",zip:12345}};
//name == "Sally"
//zip == 12345

It works with arrays too…

let [first,,third] = ["apple","orange","banana","kiwi"]
// first == "apple"
// third == "banana"

Destructuring is handy when you’ve fetched an object or array and need to use a subset of it’s properties or elements. If your webservice call returns a huge object, destructuring will help you pull out just the parts you actually care about.

Destructuring is also handy when creating mixins - objects that you wish to sprinkle functionality into by adding certain properties or functions.

Destructuring is also handy when you’re manipulating array elements.

…to use template literals in most of your string compositions.

I recommend you get in the habit of defining string literals with the backtick (`) operator. These strings are called template literals and they do some great things for us.

First, they allow us to line wrap our string literal without using any extra operators. So as opposed to the existing method…

let pet = "{" +
"name:\"Jim\"," +
"type:\"dog\"," +
"age:8" +
"}"

…we can use…

let pet = `{
name:"Jim",
type:"dog",
age:8
}`

Elegant!

…to understand the nuances of lambda (=>) functions (aka fat-arrow functions).

And it looks like I’ve saved one of the best for last, because lambdas have so dramatically increased code concision. Not to overstate it, but lambda functions delight me.

I was introduced to lambda functions in C#. I distinctly remember one day in particular asking a fellow developer to explain what they are and when you would use one. I distinctly remember not getting it. Man, I’ve written a lot of lambda functions since then!

The main offering of the lambda is, in my opinion, the concision. Concise code is readible code, grokkable code, maintainable code.

They don’t replace standard functions or class methods, but they mostly replace anonymous functions in case you’re familiar with those. I very rarely use anonymous functions anymore. They’re great for those functions you end up passing around in JavaScript, because… well, JavaScript. You use them in scenarios like passing a callback to an asynchronous function.

Allow me to demonstrate how much more concise a lambda function is.

Here’s a call to that readFile function we were using in an earlier post. This code uses a pattern where functions are explicitly defined before being passed as callbacks. This is the most verbose pattern.

fs.readFile('myfile.txt', readFileCallback);

function readFileCallback(contents) {
//do something with the contents
}

Now let’s convert that function an anonymous function to save some lines of code. This is recommended unless of course you’re paid by the line of code.

fs.readFile('myfile.txt', function(contents) {
//do something with the contents
});

Notice that the function name went away. I for one strongly dislike the first pattern. When a callback function is only used once, I feel like it belongs inline with the function call. If of course, you’re reusing a function for a callback then that’s a different story.

Now let’s go big! Or small, rather. Let’s turn our anonymous function into a lambda.

fs.readFile('myfile.txt', txt => {
//do something with the txt
})

I love it! Notice, we were able to do away with the function keyword altogether and we specified it’s argument list (in this case only a single argument) on its own. Notice too that I called that argument txt. I could have, of course, kept the name contents, but I tend to use short (often only a single letter) arguments in lambda functions to amplify the brevity. Lambda functions are very rarely complex, so this works out well.

The loss of the function name and keyword saved some characters, but lambda functions get even shorter. If a lambda contains only a single expression, the curly braces can be dropped. The expression in this case becomes the return value of the lambda.

To illustrate, let me use a new example - this one from my post on arrays in this series…

let numbers = [1,2,3,4,5,6];
let smallNumbers = numbers.filter(n => n <= 3);

In this example, n => n <= 3 is a complete lambda function. I know, concise right?! This example illustrates the value of the single letter arguments and also introduces you to the expression syntax. The body of the lambda is n <= 3. That’s an expression. It’s not a statement such as…

let n = 3;

And it’s not a block of statements such as…

{
let m = 3;
let n = 4;
let o = 5;
}

…and like I said, when the body of your lambda is a simple express, you can drop the curly braces and the expression becomes your return value.

So in the example, the .filter() function wants a function which evaluates to true or false. Our expression n <= 3 does just that, and returns the result.

There are two caveats that I’ll draw out.

First, if you have 1 argument in your lambda function, you do not need parenthesis around the argument list. In our previous example, n => n <= 3 is a good example of that. If you have 0 arguments or more than 1 argument, however, you do. These are all valid…

() => console.log('go!') //0 arguments
x => x * x //1 argument
(a,b) => a + b //2 arguments
(prefix, firstName, lastName) => `${lastName}, ${prefix} ${firstName}` //3 arguments

If you use TypeScript, you may notice that the presence of a type on a single argument lambda function requires you to wrap it with parenthesis as well, such as (x:number) => x * x.

The second caveat is when your lambda returns an expression, but that expression is an object literal wrapped in curly braces ({}). In this case, the compiler confuses your intention to return an object with an intention to create a statement block.

This, then, is not valid…

let generatePerson = (first,last) => {name:`${first} ${last}`}

To direct the compiler just do what you always did in complex mathematical statements in high school - add some more parenthesis! We could correct this as so…

let generatePerson = (first,last) => ({name:`${first} ${last}`})

And there’s one more thing about lambdas that you should know. Lambdas have a feature to remediate a common problem in JavaScript anonymous functions - the dreaded this assignment.

Anonymous functions (and named functions) in JavaScript are Objects, and as such they have a this operator that references them. Lambda functions do not. If you use this in a lambda function, chances are the sun will keep shining and the object you intended to reference will be referenced. No more _this = this or that = this or whatever else you used to use everywhere.

That’ll do it for arrays, and in fact that’ll do it for this series. If you jumped here from a search, headback to Level Up Your JavaScript Game! to see the rest of the content.

Thanks for reading and happy hacking!

Level Up Your JavaScript Game! - ES6 Modules

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See Level Up Your JavaScript Game! for related content.

Unfortunately, the whole concept of modules in JavaScript has undergone a ton of evolution and competing standards, and for a while it seems like no two JavaScript environments used modules the same way. Spending a little time figuring out exactly what’s happening goes a long way toward demystifying things.

To level up in JavaScript modules, I recommend you learn…

…to transition from Node.js’s CommonJS modules to ES6 modules.

Node has not yet fully adopted ES6 modules, but it’s coming soon. We developers can today though using a transpiler, and I recommend it. We may as well get into tomorrow’s habits today. Instead of…

const myLib = require('myLib');

…use…

import { myLib } from 'myLib';

The former strategy - CommonJS - is a well-established habit for most of us, but it’s not inherantly as capable as the latter - ES6 modules. I’m going to assume you’ve used the CommonJS pattern plenty and skip explaining its nuances, and talk only about the newer, better, faster, stronger ES6 modules.

To play with some the concepts on this page, install TypeScript.

npm i -g typescript

…to define an ES6 module and export all or part of it.

CommonJS modules are defined largely by putting some JavaScript in a separate file and then requiring it. ES6 modules are too. The differences come in how a module describes what it exports - that is what it makes available to anyone who decides to depend on it.

In ES6 modules, you put export on anything you want to export. Period. That’s easy :)

//mymodule.ts
let x = "a thing";
export let y = "another thing";
let z = "yet another thing";

In the above example, only y would be available to whoever takes a dependency on mymodule.

You can put export on variable declarations (like the let above), classes, functions, interfaces (in TypeScript), and more. Read on to see how these various exports get imported.

…to import an entire module.

To import everything a given module has to offer - all of the exports…

import * as mymodule from './mymodule';
console.log(mymodule.y);

The * indicates that we want everything and the as mymodule aliases (or namespaces) everything as mymodule. After this import, we would be free to use mymodule.y in our calling code.

…to import parts of a module.

Let’s say our module looked like this…

//mymodule.ts
export let x = "a thing";
export let y = "another thing";
export function sum(a,b) {
return a + b;
};

If we decide in our calling code that we need x and we need the sum function, then we can use…

import { x, sum } from './mymodule'
console.log(x);
console.log(sum(10,10));

Notice that we don’t need to prefix the x and sum functions. They’re in our namespace.

…to alias modules on import.

Sometimes, you want to change the name of something you import - for instance, to avoid a naming conflict…

import { x, sum as add } from './mymodule'

That’ll do it for ES6 module imports. Now head back to Level Up Your JavaScript Game! or move on to my final topic on ES6 features.

Level Up Your JavaScript Game! - Regular Expressions

See Level Up Your JavaScript Game! for related content.

I’m sorry, but there’s no way around it. You have to master regular expressions.

Regular expressions (regex for short) have a reputation of being very difficult, but if you happen to be an entry-level developer, I really don’t want you to be intimidated by them. They’re actually not so difficult. They only look difficult once you’ve created one. In a sense, they’re an easy way to at least look like a ninja.

JavaScript’s implementation of regular expressions was tough for me at first because there are a few different ways to go about it. Spend some time writing and calling a couple of patterns though, and you’ll quickly master it.

To level up in JavaScript regular expressions, I recommend you learn…

…to write your regular expression.

That’s right, first you have to learn how to write a good regex pattern. I’m not going to go into detail, but if you want some help you’re a quick web search away. I highly recommend regexr.com. It’s good not only for learning the patterns, but testing them too.

In learning patterns, you should learn about capture groups too. Defining capture groups is simple - you just put parenthesis around certain parts of your pattern. Those parts of the pattern will then be available in your matchs as independent values.

Let’s say you wanted to pull the area code out of a phone number pattern. You could use a pattern like (\d{3})-\d{3}-\d{4}. That’s obviously a very simplistic pattern that would only match US-style, 10-digit phone numbers with dashes between the groups, but notice the parenthesis around the first group. That means that that part - the area code - is going to be made available as a value for you after you execute the regex.

…to quickly tell if a pattern is detected in some text.

If you don’t need the actual matchs of the regex execution, but just want to see if there’s a match, you use <pattern>.test(<text>). For example…

/\d{3}-\d{3}-\d{4}/.test('555-123-4567') //true

In JavaScript, you put regular expressions between slashes (/) just like you put strings between quotes.

…would return true.

…to use .exec() for single pattern matches with capture groups.

If you need not only to know that the pattern matched, but also to get values from the match such as the match itself and all of the capture group values, then you use .exec()

let match = /(\d{3})-\d{3}-\d{4}/.exec('555-123-4567');
match[0] //555-123-4567
match[1] //555

…and because I added parenthesis around the first number group there, that value should be returned as part of the match. The match itself is always the first match ([0]), and each subsequent capture group in the order you defined them from left to right follow ([1], [2], …, [n]).

…to use .match() to find multiple matches in a string.

The .match() function is on String.prototype, so it’s available on any string. Besides flipping the calling pattern from .exec() (.exec() uses <pattern>.exec(<text>) while .match() uses <text>.match(<pattern>)), this function has a couple of other peculiarities.

First, it does not capture from your capture groups, so if that’s what you’re looking to do, then use .exec().

Second, it is capable of capturing multiple matches returned as an array. So if you do something like…

"14 - 8 = 6".match(/\d+/g) //[14,8,6]

The g stands for global and is a regex option that tells it to look in the entire string. Look at all of the other options that are valid there too. They can be helpful.

If you need to capture multiple matches (like you get with .match()), but you also want the capture groups (like you get with .exec()), then you need to call .exec() in a loop like this…

let text = "The quick brown fox jumps over the lazy dog.";
let match;
while (match = /(t)he/ig.exec(text)) {
console.log(match[0]);
console.log(match[1]);
}

/* Should log...
The
T
the
t
*/

Note that I included an i and a g option on the regex (/the/). The i makes the search case insensitive and the g directs it to find every match in the text. Notice that match[0] equals the full match each iteration and match[1] is the contents of the capture group I defined (the first letter of the word “the” for whatever reason).

That’ll do it for regular expressions. Now head back to Level Up Your JavaScript Game! or move on to the next topic on ES6 module imports.

Level Up Your JavaScript Game! - Arrays

See Level Up Your JavaScript Game! for related content.

Working with JavaScript arrays is practically an everyday task.

Arrays are simply collections of things, and we often find need to perform some function to each of their items or perhaps to subsets of their items.

Years ago, ES5 introduced a bunch of new array functions that you should be or become familiar with. The three I’ll highlight are filter, map, and reduce.

To level up in JavaScript arrays, I recommend you learn…

…to use the .filter() function to reduce an array down to a subset.

This is not a difficult topic, but it’s an important one. If you have a set of numbers [1,2,3,4,5,6] and you’d like to limit it to numbers less than or equal to 3, you would do…

let numbers = [1,2,3,4,5,6];
let smallNumbers = numbers.filter(n => n <= 3);

Take note of what the fact that .filter() hangs off of an array. It is in fact a function on Array.prototype and is thus available from every array. So [].filter is valid.

.filter() asks for a function with a single argument that represents a single item in the array. The .filter() function is going to execute the function you give it on each and every item in the array. If your function returns true, then it’s going to include that item in the resulting set. Otherwise it won’t. In the end, you’ll have a subset of the array you called .filter() on.

This brings up something I see a lot in folks that have been programming a while.

Imagine this common pattern…

let people = [
{id:1, name:"Jill", age:34, gender:"female"},
{id:2, name:"John", age:42, gender:"male"},
{id:3, name:"Jane", age:19, gender:"female"},
{id:4, name:"Jake", age:31, gender:"male"},
];
for(let i = 0; i < people.length; i++) {
if(people[i].age < 40 && people[i].gender == "female") {
fetch("http://mywebservice.com/api/ordersByPeopleId/" + people[i].id)
.then((results) = {
//do something with results for Jill and Jane
});
}
}

What’s wrong with that code? Well, it works, so there’s nothing functionally wrong with it. It’s too verbose though. If we use some array functions, we could drastically increase the readibility and maintainability. Let’s try…

people.forEach(p => {
if(p.age < 40 && p.gender == "female") {
fetch("http://mywebservice.com/api/ordersByPeopleId/" + people[i].id)
.then((results) = {
//do something with results for Jill and Jane
});
}
})

Here, we replaced the for loop with a forEach array function that we hang right on our array. This allows us to refer, inside our loop, to simply p instead of people[i]. I love this. I find for loops difficult and unnatural to write.

Some argue against using single-letter variables like p and would prefer to call that something like person. Do what makes you happy and works well with your team, but I like single-letter variables inside of fat-arrow functions where concision is king.

Lets do another round…

people
.filter(p => p.age < 40 && p.gender == "female")
.forEach(p => {
fetch("http://mywebservice.com/api/ordersByPeopleId/" + people[i].id)
.then((results) = {
//do something with results for Jill and Jane
});
})

Here, we pulled the if statement out of our loop and added it as a .filter() function before our .forEach() function in a chain of array functions. This effectively separates the logic we use for filtering with the logic we which to take effect on our subset of people - a very good idea.

I might even take the separation of .filter() a step further and do…

people
.filter(p => p.age < 40)
.filter(p => p.gender == "female")
.forEach(p => {
fetch("http://mywebservice.com/api/ordersByPeopleId/" + people[i].id)
.then((results) = {
//do something with results for Jill and Jane
});
})

To me, that’s more clear.

…to use the .map() function to transform elements in an array.

Think of arrays, for a second, like you do database tables. An array entry is analogous to a database table’s row, and an array property is analogous to a database table’s column.

In this analogy, the .filter() function reduces the rows, and the .map() function which I’d like to talk about now reduces (potentially) the columns - more generally, it transforms the element.

That transformation is entirely up to you and it can be severe. You might do something simple like pull a person’s name property out because it’s the only one you’re concerned with. You might just as well do something more complex like transform each person to a web service call and the resulting promise. Let’s try that with our previous code…

let orderPromises = people
.filter(p => p.age < 40)
.filter(p => p.gender == "female")
.map(p => fetch(`http://mywebservice.com/api/ordersByPeopleId/${p.id}`)
})

Notice that now, each of the females under 40 is fetched from a webservice. The fetch() function returns a promise, so each array item is transformed from a person object to a promise. After the run, orderPromises is an array of promises. By the way, you could then execute code after all orders have been retrieved, using…

let ordersByPerson = await Promise.all(orderPromises);
//do something with ordersByPerson

…to use reduce to turn an array into some scalar value.

If you really want to be a JavaScript ninja, don’t miss the .reduce() array function and it’s zillion practical uses!

As opposed to .map() which acts on each element in an array and results in a new array, .reduce() acts on each element in an array and results in a scalar object by accumulating a result with each step.

For example, if you have an array of orders and you want to calculate sales tax on each order based on total and location, you would use .map() to turn arrayOfOrders into arrayOfOrdersWithSalesTax (start with an array and end with an array).

let arrayOfOrdersWithSalesTax = arrayOfOrders
.map(o => ({...o, salesTax: calculateSalesTax(o.total,o.location) }));

 

The .map() function in the preceding example uses an object spread operator (…) to tack another property onto each order item. You can read more about the spread operator in my Level Up Your JavaScript Game! - ES6 Features post.

If, however, you wanted to calculate the total sales tax for all orders, you would use .reduce() to turn arrayOfOrders into totalSalesTax (start with an array and end with a scalar).

let totalSalesTax = arrayOfOrdersWithSalesTax
.reduce((a,o) => { a += calculateSalesTax(o.total, o.location); }, 0);

It’s not immediately apparent how that reduce function works, so let me walk you through it.

The .reduce() function asks for a function with 2 arguments - an accumulator which I’m calling a and a current which I’m calling o because I know that my current item on each loop is actually an order. This makes it clear to me in my function that o means order. Finally, the reduce function itself takes a second argument - the initial state. In my sample, I’m using 0. Before we’ve added up any sales tax, our total sales tax should be 0, right?

The function you pass in to .reduce() then executes for each item in the array and by our definition, it calculates the sales tax and adds (or accumulates) the result to the a object. When the .reduce() function has completed its course, it returns the value of a, and my code saves that in a new local variable calle3d totalSalesTax.

Pretty cool, eh?

Let me be clear that I said that .reduce() turns an array into a scalar, but that scalar can most anything you want including a new array.

That’ll do it for arrays. Now head back to Level Up Your JavaScript Game! or move on to the next topic on regular expressions.

Level Up Your JavaScript Game! - Asynchrony

See Level Up Your JavaScript Game! for related content.

Most any JavaScript application you touch now uses asynchrony, so it’s a critical concept although it’s not a simple one.

I usually start any discussion on asynchrony by clarifying the difference between asynchrony and concurrency. Concurrency is branching tasks out to separate threads. That’s not what we’re talking about here. We’re talking here about asynchrony which is using a single thread more efficiently by basically using the gaps where we were otherwise frozen waiting for a long process.

One of the tough things about asynchrony in JavaScript is all the options that have emerged over time. Options are a double-edged sword. It’s both good and bad to have 20 different ways to accomplish a task.

To level up in JavaScript asynchrony, I recommend you learn…

…to call a function that returns a promise.

This is the most basic thing to understand about promises. How to call a function that returns one and determine what happens when the promise resolves.

To review, calling a regular (synchronous) function goes…

let x = f();
function f() {
//do something... even if it takes a while
return "answer";
}
//x = answer

And the problem is that if f takes a while, then the thread is blocked and you don’t get to be more efficient and do work in the meantime.

The solution is returning from f with a “place holder” - called a Promise - immediately and then “resolving” it when the work is done (or “rejecting” it if there’s an exception). Here’s what that looks like…

let x = f().then(() => {
//do something after the function is done
};

//do something even before the function comes back

function f() {
//do something that takes a while, but return a promise immediately
}

One more thing. When a promise is resolved, it can contain a payload, and in your .then() function you can simply define an argument list in your handler function to get that payload…

let x = f().then(payload => {
//do something... payload is available
});

Luckily, a lot of functions already return promises. If you want to read a file using the fs module in Node, for instance, you call fs.readFile() and what you get back is a promise. Again, it’s the simplest case for asynchrony, and here’s what that would look like…

const fs = require('fs');
fs.readFile('myfile.txt').then(file => {
//do something with file
});
//do something even before `fs.readFile` comes back from accessing a file on disk and reading its contents

…to write a function that passes on a promise.

If the simplest case for asynchrony is calling functions that return promises, then the next step is defining your own function which passes a promise on. Recall the example I used where we wanted to use fs.readFile. Well, what if we wanted to refactor our code and put that function call into our own function.

It’s important to realize that it’s rarely sensible to create a sychronous function which itself calls an asychronous function. If your function needs to do something internally that is asynchronous, then you very likely want to make your function itself asynchronous. How? By passing on a promise.

Let’s write that function for reading a file…

getFileText('myfile.txt').then(file => {
//do something with file
})

function getFileText(name) {
return fs.readFile(name);
}

Easy, eh? If fs.readFile returns a promise, then we can return that promise to our caller. By definition, if our function returns a promise, then it’s an asynchronous function.

…to write a function that creates and returns a promise.

But what if you want to create an asynchronous function that itself doesn’t necessarily call a function that returns a promise? That’s where we need to create a new promise from scratch.

As an example, let’s look at how we would use setTimeout to wait for 5 seconds and then return a promise. The setTimeout function in JavaScript (both in the browser and in Node) is indeed asynchronous, but it does not return a promise. Instead it takes a callback. This is an extremely common pattern in JavaScript. If you have a function that needs to call another function that wants a callback, then you need to either keep with the callback pattern (no thank you) or essentially transform that callback pattern into a promise pattern. Let’s go…

waitFive().then(() => {
//do something after 5 seconds
})
//do something immediately... this will execute first

function waitFive() {
return new Promise((resolve,reject) => {
setTimeout(() => {
resolve();
}, 5000);
});
}

See how the first statement in the waitFive function is a return. That lets you know that function is going to come back with an answer immediately. Within the new Promise() call we pass in a handler - a function that takes 2 arguments: resolve and reject. In the body of our handler, resolve and reject are not static variables - they’re functions, and we call them when we’re done, either because things went well or they didn’t. It’s just super neat that we’re able to call them from inside of a callback. This is possible due to the near magic of JavaScript closure.

…to chain promises and catch exceptions.

You should be sure you understand how promise chaining is done. Chaining is a huge advantage to the promise pattern and it’s great for orchestrating global timing concerns in your application - i.e. first I want this to happen and then this and then this.

Here’s what a chain looks like…

f()
.then(() => { /* do something */ })
.then(() => { /* do something */ })
.then(() => { /* do something */ })

…where each of those handlers that we’re passing to the .then() functions can have payloads.

There’s some wizardry that the .then() function will do for us as well. It will coerce the return value of each handler function so that it returns a promise every time! Watch this…

f()
.then(() => { return "foo"; })
.then((result) => { /* result = foo */ })

Pay close attention to what’s happening here. The first .then() is returning a string, but we’re able to hang another .then() off of it. Why? Because .then() coerced "foo" into a promise with a payload of "foo". This is the special sauce that allows us to chain.

There’s a shortcoming with promises here by the way. Let me set it up…

f()
.then(() => { return "value 1"; })
.then((value1) => {
//do something with value1
return "value2";
})
.then((value2) => {
//PROBLEM: value2 is available, but value 1 is not
})

The unfortunate remedy to this problem is…

let v1;

f()
.then(() => { return "value 1"; })
.then((value1) => {
//do something with value1
v1 = value1
return "value2";
})
.then((value2) => {
//value2 is available, and value 1 is available as v1
})

That’s a bit hacky, but it’s a problem that’s solved very elegantly by async/await coming up.

…to save a promise so you can check with it at any point and see if it’s been resolved.

This is great for coordinating timing in a complex application.

This is a little trick that I use quite a bit, though I don’t think it’s very common. It’s quite cool though and I don’t see any drawbacks.

let ready = f();

ready.then(() => { /* do something */ });

...

ready.then(() => { /* do something */ });

...

ready.then(() => { /* do something */ });

What I’m doing is saving the result of my function call to a variable and then calling .then() on it any time I want throughout my codebase.

You might wonder why this is necessary. Wouldn’t the first call be the only one that needs to “wait” for the promise? Actually, no. If you’re creating code that must not run until f() is done, then you need to wait for it. It’s very likely that subsequent references to the promise happen when the promise is already resolved, but that’s fine - your handler code will simply run immediately. This just assures that that thing (f() in this case) has been done first.

…to write an asynchronous function using async instead of creating a promise and calling it using await instead of .then().

The async/await pattern is one that some clever folks at Microsoft came up with some years ago in C#. It was and is so great, that it’s made its way into other languages like JavaScript. It’s a standard feature in the most recent versions of Node.js, so it’s ready for you out of the box.

In JavaScript, async and await still use promises. They just make it feel good.

For defining the asynchronous function, instead of…

function f() {
return new Promise((resolve, reject) => {
//do something that takes a while
resolve("result");
})
}

…you do…

async function f() {
//do something that takes a while
return "result";
}

And the angels rejoice! That’s way more understandable code.

Likewise, on the calling side, instead of…

f().then(result => {
//do something with result
});

…you do…

let result = await f();

Yay! How great is that.

It seems odd at first, but the statements that come after the line with await do not execute until after f() comes back with its answer. I like to mentally envision those statements as being inside of a callback or a .then() so I understand what’s happening.

As I eluded to before, this solves that nasty little problem we had with the promise calling pattern…

let value1 = await f1();
//do something... value1 is available

let value2 = await f2(value1);
//do something... value1 and value2 are available

async function f1() { return "value 1"; }
async function f2(value1) {
//do something with value1
return "value 2";
}

Notice that I was a bit more verbose in that I defined f2. I didn’t have to, but the code is far more readable and more importantly, value1 is available not only inside of f2, but also between the function calls and after both.

Very cool.

…to understand the difference between each of the following lines of code.

let x = f;
let y = f();
let z = await f();

The differences may not be obvious at first.

The first line makes x to be the asynchonous function that f is. After the first line executes, you would be able to call x().

The second executes f() and sets y equal to the resulting promise. After the second line executes, you would be able to use y.then() or await y to do something after f() resolves.

The third executes f() and sets z equal to the payload of the promise returned by f().

Let me finally add one random tidbit, and that is that you should understand that the async operator can be added to a fat arrow function just as well as a normal function. So you may write something like…

setTimeout(async () => {
let results = await fetch("http://mywebservice.com/api/widgets");
return results;
})

You can’t use await except inside of a function marked with async.

If you find yourself trying to call await but you’re not in an async function, you could do something like this…

(async () => {
//use await
})();

That simply declares and invokes a function that’s marked as async. It’s a bit odd, but it works a treat.

That’ll do it for asynchrony. Now head back to Level Up Your JavaScript Game! or move on to the next topic on arrays.

Level Up Your JavaScript Game!

A fellow developer recently expressed a sentiment I’ve heard and felt many times myself.

“There are a lot of JavaScript concepts I know, but I don’t think I could code them live in front of you right now.”

It’s one thing to understand the concept of a Promise or destructuring in JavaScript, but it’s quite another to be able to pull the code out of your shiver without a web search or a copy/paste.

There are so many concepts like this for me as a developer. They’re my gaps - the pieces I know are missing. I know they won’t take long to fill, but it’s just a matter of finding and making the time. My strategy is to…

  1. Record them
    As I become aware of these gaps, I write them on my task list. I may not get to them right away, and that’s fine. When I have a spare hour though, I turn to these items in my task list and then off I go, learning something new.

  2. Write into permanent memory storage
    Computers can save things permanently with a single write. For me, it takes 4 or 5 writes. For example, a long time ago, I wanted to learn how to write a super basic web server in Node.js - from memory. So I looked it up and found something like this…

    var html = require('http');
    html.createServer((req,res) => {
    res.end('hi')
    }).listen(3000)

    I found it, tried to memorize it, tried to write it from memory, failed, looked it up, and tried again as many times as it took until I could. Now I have it. I can whip it up in a hurry if I’m trying to show basic Node concepts to someone.

In counseling my friend on what JavaScript concepts would be beneficial to practice, I decided to compose this rollup blog post called Level Up Your JavaScript Game! to share more broadly.

There are 5 things I recommend you not only grok generally, but know deeply and can whip up on request…

  1. Promises and Async/Await
  2. Manipulating an array
  3. Regular Expressions
  4. ES6 Module
  5. Other ES6 Language Features

NPM Link

My buddy Jason Young (@ytechie) asked a question the other day that reminded me of a Node trick I learned sometime ago and remember getting pretty excited about.

First, let’s define the problem.

If you are working on a Node project and you want to include an npm package as a dependency, you just install it, require it, and then do a fist pump.

If, however, you are in one of the following scenarios…

  • You find a great package on npm, but it’s not exactly what you want, so you fork it on GitHub and then modify it locally.

  • You are working on a new awesome sauce npm package, but it’s not done yet. But you want to include it in a node project to test it while you work on it.

…then you’re in a pickle.

The pickle is that if in your consuming app, you’ve done a npm install my-awesome-package then that’s the version from the public registry.

The question is, how do you use a local version.

There are (at least) two ways to do it.

The first is to check your project (the dependency npm package that you’ve forked or you’re working on) in to GitHub and then install it in your consuming project using npm install owner/repo where owner is your GitHub account. BTW, you might want to npm remove my-awesome-package first to get rid of the one installed from the public registry.

This is a decent strategy and totally appropriate at times. I think it’s appropriate where I’ve forked a package and then want to tell my friend to try my fork even though I’m not ready to publish it to npm yet.

I don’t want to expound on that strategy right now though. I want to talk about npm’s link command (documentation).

The concept is this. 1) You hard link the dependency npm package into your global npm package store, and 2) you hard link that into your consuming project.

It sounds hard, but it’s dead simple. Here’s how…

  1. At your command line, browse to your dependency package’s directory.
  2. Run npm link
  3. Browse to your consuming project’s directory.
  4. Uninstall the existing package if necessary using npm remove my-awesome-package
  5. Finally, run npm link my-awesome-package

You’ll notice that the link isn’t instant and that will cause you to suspect that it’s doing more than just creating a hard link for you, and you’re right. It’s doing a full package install (and a build if necessary) of the project.

The cool part is that since the project directory is hard linked, you can open my-awesome-package in a new IDE instance and work away on it and when you run the consuming project, you’ll always have the latest changes.

And that’s that. I use this trick all the time now that I know it. Before I knew it, you’d see version counts like 1.0.87 in my published packages because I would roll the version and republish after every change. Oh, the futility!

The inverse is just as easy. When the latest my-awesome-package has been published to npm and you’re ready to use it, just visit your consuming package and run npm unlink my-awesome-package and then npm install my-awesome-package. Then go to your dependency package and simply run npm unlink. Done.

The Regatta Project

My role at Microsoft is that of an evangelist, and that implies that we speak broadly to audiences about technology. Recently, however, my team is working one-on-one with organizations on exciting new Microsoft technologies - making sure they’re exactly what developers need.

One of the projects I’ve been working on recently is called Regatta championed by Baiyin Zhou (@baiyinvc) from Boston.

This entire idea is super fun. Imagine exercising on a rowing machine in a studio in a group session. Now imagine a visual on the wall depicting your rowing machine and those of the other session participants as real boats. As you heave and ho, you see your little boat moving across the screen.

There are just so many awesome scenarios that are possible at this point.

The visual of the boats alone is encouragement for you to work harder to compete against others in the class. But that’s pretty obvious isn’t it. What about the less obvious scenarios? What about being able to add in a phantom boat that represents the group’s average from yesterday’s workout? Now you’re not competing against your fellow rowers - you’re competing with them. You have to beat that phantom boat!

As soon as things in the real world - like our efforts to stay in shape - are digitized and turned into data, we get to do things like capture our progress, integrate with other fitness tracker systems, or even do data analysis to determine some deep learning from it.

This is the type of concept where digital systems really have an opportunity to affect our lives for the better. I keep saying that technology is not so interesting unless it improves our lives. For being such a technophile, I’m actually quite the skeptic, because all too often technology just gets in the way.

The Regatta Project envisions a future with a whole lot of rowing machines deployed with smart devices (the Raspberry Pi Zero at this point) attached to them, and assuring good communication with all of these devices is a great job for Azure IoT Hub.

Go check out the details of the [Regatta](https://microsoft.github.io/techcasestudies/iot/2016/12/10/regatta.html) project and let me know what you think with a comment below.

Regatta Architecture

npm Erroring Out on Windows 10 Fast Ring Build 14367

Just in case someone else is running into the same thing, I’m running Windows 10 Insiders Build 14367 (on the fast ring), and I’m unable to use npm. When I do a simple npm -v I get this error…

C:\code>npm -v

events.js:160
throw er; // Unhandled 'error' event
^

Error: This socket is closed
at WriteStream.Socket._writeGeneric (net.js:672:19)
at WriteStream.Socket._write (net.js:724:8)
at doWrite (_stream_writable.js:307:12)
at writeOrBuffer (_stream_writable.js:293:5)
at WriteStream.Writable.write (_stream_writable.js:220:11)
at WriteStream.Socket.write (net.js:650:40)
at WriteStream.stream.write (C:\Program Files\nodejs\node_modules\npm\node_modules\npmlog\node_modules\ansi\lib\newlines.js:36:21)
at Cursor.write (C:\Program Files\nodejs\node_modules\npm\node_modules\npmlog\node_modules\ansi\lib\ansi.js:157:23)
at Cursor.(anonymous function) [as show] (C:\Program Files\nodejs\node_modules\npm\node_modules\npmlog\node_modules\ansi\lib\ansi.js:226:26)
at Object.ProgressBar.hide (C:\Program Files\nodejs\node_modules\npm\node_modules\npmlog\node_modules\gauge\progress-bar.js:101:15)

3.9.5

My workaround is simply to type bash and get into Ubuntu where I have node and npm. There’s not really any disadvantage to this either since it operates on the same directory structure. I’m loving this Bash on Ubuntu on Windows thing.

Deploying TypeScript Projects to Azure from GitHub Using Continuous Deployment

I’m working on a fun project called Waterbug. You can peek or play at github.com/codefoster/waterbug.

Waterbug is an app that collects data as you row on a WaterRower and visualizes it in an Angular 2.0 app.

It’s a fun app because it uses a lot of modern stuff. Modern stuff is usually the fun stuff, and that’s why it’s always nice to be working on a greenfield project.

So, like I mentioned, one of the components of this app uses Angular 2.0. Angular is itself written in TypeScript, and you’re strongly encouraged to write your Angular 2.0 apps using TypeScript. You don’t have to, but at least in my opinion, you’d be crazy not to.

TypeScript is awesome.

TypeScript makes everything more terse, more elegant, and easier to read, and it allows your tooling (Visual Studio Code is my editor of choice) to reason about your code and thus help you out immensely.

The important thing to remember about TypeScript and the reason I think for it’s rapid uptake is that it’s not a different language that compiles to JavaScript. It’s a superset of JavaScript. That means you don’t throw any of your existing work away. You just start sprinkling in TypeScript where it benefits you. If you’re like me though, it won’t be long before you’re addicted to using it everywhere.

When you’re working on a TypeScript project, you write in .ts files and those get transpiled from .ts files to .js files.

Herein lies our first question.

Should we check those .js files (and also the .js.map files that are created by default) into our code repository (GitHub in my case)?

The answer is _no_.

The .js code is derivative and does not belong in source control. Source control is for source files. The .ts files are our source files in this case.

If you start checking your .js files into source control, you’re inevitably going to end up with .ts files and their associated .js files out of sync. Hair pulling will surely ensue.

I’ve gone one step further and determined that I don’t even want to look at my .js files in my editor.

In Visual Studio Code, I can go to File | Preferences | Workspace Settings, which opens (or creates if necessary) my projects .vscode\settings.json file. Then I can sprinkle in a little magic dust and tell Code that I’m not so concerned with .js and .js.map files and I’d just rather they not show up in my File Explorer pane or in my global search results.

Here’s the magic dust…

{
"files.exclude": {
"app/**/*.js": true,
"app/**/*.js.map": true
}
}

If, however, you don’t check your .js files into GitHub, then when you configure Azure to do continuous deployment from GitHub, it’s not going to pull in any .js files and that’s what your users’ browsers really need to make the site run.

So this is where some people say “Oh, blasted! I’ll just check my .js files in and call it done”.

True that works, but it also incurs technical debt. Don’t do it. It’s not worth it. Stick to your philosophical guns and don’t make choices like this. It may cost a little more up front to figure out the right way, but you’ll be glad later.

So, where and when should the .ts files get transpiled?

The answer is that they should get transpiled in Azure and it should happen each time there’s a deployment.

Now, let’s dig in and figure out how to do this.

If you do a little research, you’ll find that when you wire Azure up to look at GitHub, it does a pull of the code every time you push to the configured branch. Then it runs a default deployment script if you haven’t specified otherwise.

To run some code for each deployment, you simply customize this deployment script. You do that by adding two files to the root of your project: .deployment and deploy.cmd. You could just create these files manually, of course, but it’s better to generate them. That way you have the latest recommended default script and it specifically made for the type of application you’re running.

To generate the default deployment script, you first need to have the Azure Xplat CLI tool installed, which is a breeze. Just do npm install -g azure-cli. If you already have it and haven’t updated it for a while, then run npm up -g azure-cli.

After you have the azure-cli tool, you need to login to your Azure subscription. This is a lot easier than it used to be.

Simply type azure login. That will generate a little code for you and then ask you to go to a website, login, and enter your code. From that point forward, you’re able to access your Azure goodies from your command line. CLI FTW!

Once you get that, just go to the root of your website project (at the command line) and then run…

azure site deploymentscript --node

This will create the .deployment and deploy.cmd files.

Okay, now we just have to customize the deploy.cmd file a bit.

If your deployment script looks like mine, then there’s a part that looks like this…

:: 3\. Install npm packages

IF EXIST "%DEPLOYMENT_TARGET%\package.json" (
pushd "%DEPLOYMENT_TARGET%"
call :ExecuteCmd !NPM_CMD! install --production
IF !ERRORLEVEL! NEQ 0 goto error
popd
)

That script runs npm install to install your npm dependencies. It adds the --production flag to indicate that developer dependencies should be skipped since this is not a dev box - it’s the real deal!

Just after an npm install, you’re ready for the meat of the matter. It’s time to turn all of your .ts files into .js files.

To accomplish this, I added this just after step 3…

:: 4\. Compile TypeScript
echo Transpiling TypeScript in %DEPLOYMENT_TARGET%...call :ExecuteCmd node %DEPLOYMENT_TARGET%\node_modules\typescript\bin\tsc -p "%DEPLOYMENT_TARGET%"

The first line is obviously a comment.

The echo shows what’s going on in the console so you can find it in the log files and such.

The last line calls :ExecuteCmd (which is a function that comes with the default deployment script) and asks it to run TypeScript’s commandline compiler (tsc) using node and pointing it to the deployment target. The deployment target is the /site/wwwroot directory that contains your site. The command explicitly uses the tsc command that’s in the deployment target’s node_modules\typescript\bin folder. That should be there because we have typescript defined as one of the projects dependencies in the package.json. Therefore the npm install from a few lines up should have installed typescript. Another strategy would be to install typescript globally, but I opted for this method.

And that’s really all there is to it. I like to jump over to my SCM site (.scm.azurewebsites.net) and go to Debug Console | PowerShell to see the actual files on the site and make sure the .js files were generated.

If you look in the list of deployments in your Azure portal, you can actually double-click on the latest deployment and then click on View Log to see the console output that was captured when this deployment script ran…

In the log, you can see our echo and that the transpilation process has occurred. Don’t worry about the errors that are thrown. Those are expected and didn’t stop the process from completing.