Unlocking the JavaScript Core

Dmitry Sheiko

February 2016

You may have owned an iPhone for years and regard yourself as an experienced user. At the same time, you keep removing unwanted characters one at a time while typing by pressing delete. However, one day you find out that a quick shake allows you to delete the whole message in one tap. Then you wonder why on earth you didn't know this earlier. The same thing happens with programming. We can be quite satisfied with our coding until, all of sudden, we run into a trick or a lesser-known language feature that makes us reconsider the entire work done over the years. It turns out that we could do this in a cleaner, more readable, more testable, and more maintainable way. So it's presumed that you already have experience with JavaScript; however, this article equips you with the best practices to improve your code.

(For more resources related to this topic, see here.)

We will cover the following topics:

  • Making your code readable and expressive
  • Mastering multiline strings in JavaScript
  • Manipulating arrays in the ES5 way
  • Traversing an object in an elegant, reliable, safe, and fast way
  • The most effective way of declaring objects
  • How to magic methods in JavaScript

Make your code readable and expressive

There are numerous practices and heuristics to make a code more readable, expressive, and clean. We will cover this topic later on, but here we will talk about syntactic sugar. The term means an alternative syntax that makes the code more expressive and readable. In fact, we already had some of this in JavaScript from the very beginning. For instance, the increment/decrement and addition/subtraction assignment operators inherited from C. foo++ is syntactic sugar for foo = foo + 1, and foo += bar is a shorter form for foo = foo + bar. Besides, we have a few tricks that serve the same purpose.

JavaScript applies logical expressions to so-called short-circuit evaluation. This means that an expression is read left to right, but as soon as the condition result is determined at an early stage, the expression tail is not evaluated. If we have true || false || false, the interpreter will know from the first test that the result is true regardless of other tests. So the false || false part is not evaluated, and this opens a way for creativity.

Function argument default value

When we need to specify default values for parameters we can do like that:

function stub( foo ) {
return foo || "Default value";
}

console.log( stub( "My value" ) ); // My value
console.log( stub() ); // Default value

What is going on here? When foo is true (not undefined, NaN, null, false, 0, or ""), the result of the logical expression is foo otherwise the expression is evaluated until Default value and this is the final result.

Starting with 6th edition of EcmaScript (specification of JavaScript language) we can use nicer syntax:

function stub( foo = "Default value" ) {
return foo;
}

Conditional invocation

While composing our code we shorten it on conditions:"

var age = 20;
age >= 18 && console.log( "You are allowed to play this game" );
age >= 18 || console.log( "The game is restricted to 18 and over" );

In the preceding example, we used the AND (&&) operator to invoke console.log if the left-hand condition is Truthy. The OR (||) operator does the opposite, it calls console.log if the condition is Falsy.

I think the most common case in practice is the shorthand condition where the function is called only when it is provided:

/**
* @param {Function} [cb] - callback
*/
function fn( cb ) {
cb && cb();
};

The following is one more example on this:

/**
* @class AbstractFoo
*/
AbstractFoo = function(){
// call this.init if the subclass has init method
this.init && this.init();
};

Syntactic sugar was introduced to its full extent to the JavaScript world only with the advance in CoffeeScript, a subset of the language that trans-compiles (compiles source-to-source) into JavaScript. Actually CoffeeScript, inspired by Ruby, Python, and Haskell, has unlocked arrow-functions, spreads, and other syntax to JavaScript developers. In 2011, Brendan Eich (the author of JavaScript) admitted that CoffeeScript influenced him in his work on EcmaScript Harmony, which was finalized this summer in ECMA-262 6th edition specification. From a marketing perspective, the specification writers agreed on using a new name convention that calls the 6th edition as EcmaScript 2015 and the 7th edition as EcmaScript 2016. Yet the community is used to abbreviations such as ES6 and ES7. To avoid confusion further in the book, we will refer to the specifications by these names. Now we can look at how this affects the new JavaScript.

Arrow functions

Traditional function expression may look like this:

function( param1, param2 ){ /* function body */ }

When declaring an expression using the arrow function (aka fat arrow function) syntax, we will have this in a less verbose form, as shown in the following:

( param1, param2 ) => { /* function body */ }

In my opinion, we don't gain much with this. But if we need, let's say, an array method callback, the traditional form would be as follows:

function( param1, param2 ){ return expression; }

Now the equivalent arrow function becomes shorter, as shown here:

( param1, param2 ) => expression

We may do filtering in an array this way:

// filter all the array elements greater than 2
var res = [ 1, 2, 3, 4 ].filter(function( v ){
return v > 2;
})
console.log( res ); // [3,4]

Using an array function, we can do filtering in a cleaner form:

var res = [ 1, 2, 3, 4 ].filter( v => v > 2 );
console.log( res ); // [3,4]

Besides shorter function declaration syntax, the arrow functions bring the so called lexical this. Instead of creating its own context, it uses the context of the surrounding object as shown here:

"use strict";
/**
* @class View
*/  
let View = function(){
let button = document.querySelector( "[data-bind=\"btn\"]" );
/**
* Handle button clicked event
* @private
*/
this.onClick = function(){
   console.log( "Button clicked" );
};
button.addEventListener( "click", () => {
   // we can safely refer surrounding object members
   this.onClick();
}, false );
}

In the preceding example, we subscribed a handler function to a DOM event (click). Within the scope of the handler, we still have access to the view context (this), so we don't need to bind the handler to the outer scope or pass it as a variable through the closure:

var that = this;
button.addEventListener( "click", function(){
// cross-cutting concerns
that.onClick();
}, false );

Method definitions

As mentioned in the preceding section, arrow functions can be quite handy when declaring small inline callbacks, but always applying it for a shorter syntax is controversial. However, ES6 provides new alternative method definition syntax besides the arrow functions. The old-school method declaration may look as follows:

var foo = {
bar: function( param1, param2 ) {
}
}

In ES6 we can get rid of the function keyword and the colon. So the preceding code can be put this way:

let foo = {
bar ( param1, param2 ) {
}
}

The rest operator

Another syntax structure that was borrowed from CoffeeScript came to JavaScript as the rest operator (albeit, the approach is called splats in CoffeeScript).

When we had a few mandatory function parameters and an unknown number of rest parameters, we used to do something like this:

"use strict";
var cb = function() {
// all available parameters into an array
var args = [].slice.call( arguments ),
     // the first array element to foo and shift
     foo = args.shift(),
     // the new first array element to bar and shift
     bar = args.shift();
console.log( foo, bar, args );
};
cb( "foo", "bar", 1, 2, 3 ); // foo bar [1, 2, 3]

Now check out how expressive this code becomes in ES6:

let cb = function( foo, bar, ...args ) {
console.log( foo, bar, args );
}
cb( "foo", "bar", 1, 2, 3 ); // foo bar [1, 2, 3]

Function parameters aren't the only application of the rest operator. For example, we can use it in destructions as well, as follows:

let [ bar, ...others ] = [ "bar", "foo", "baz", "qux" ];
console.log([ bar, others ]); // ["bar",["foo","baz","qux"]]

The spread operator

Similarly, we can spread array elements into arguments:

let args = [ 2015, 6, 17 ],
   relDate = new Date( ...args );
console.log( relDate.toString() ); // Fri Jul 17 2015 00:00:00 GMT+0200 (CEST)

Mastering multiline strings in JavaScript

Multi-line strings aren't a good part of JavaScript. While they are easy to declare in other languages (for instance, NOWDOC), you cannot just keep single-quoted or double-quoted strings in multiple lines. This will lead to syntax error as every line in JavaScript is considered as a possible command. You can set backslashes to show your intention:

var str = "Lorem ipsum dolor sit amet, \n\
consectetur adipiscing elit. Nunc ornare, \n\
diam ultricies vehicula aliquam, mauris \n\
ipsum dapibus dolor, quis fringilla leo ligula non neque";

This kind of works. However, as soon as you miss a trailing space, you get a syntax error, which is not easy to spot. While most script agents support this syntax, it's, however, not a part of the EcmaScript specification.

In the times of EcmaScript for XML (E4X), we could assign a pure XML to a string, which opened a way for declarations such as these:

var str = <>Lorem ipsum dolor sit amet,
consectetur adipiscing
elit. Nunc ornare </>.toString();

Nowadays E4X is deprecated, it's not supported anymore.

Concatenation versus array join

We can also use string concatenation. It may feel clumsy, but it's safe:

var str = "Lorem ipsum dolor sit amet, \n" +
"consectetur adipiscing elit. Nunc ornare,\n" +
"diam ultricies vehicula aliquam, mauris \n" +
"ipsum dapibus dolor, quis fringilla leo ligula non neque";

You may be surprised, but concatenation is slower than array joining. So the following technique will work faster:

var str = [ "Lorem ipsum dolor sit amet, \n",
"consectetur adipiscing elit. Nunc ornare,\n",
"diam ultricies vehicula aliquam, mauris \n",
"ipsum dapibus dolor, quis fringilla leo ligula non neque"].join( "" );

Template literal

What about ES6? The latest EcmaScript specification introduces a new sort of string literal, template literal:

var str = `Lorem ipsum dolor sit amet, \n
consectetur adipiscing elit. Nunc ornare, \n
diam ultricies vehicula aliquam, mauris \n
ipsum dapibus dolor, quis fringilla leo ligula non neque`;

Now the syntax looks elegant. But there is more. Template literals really remind us of NOWDOC. You can refer any variable declared in the scope within the string:

"use strict";
var title = "Some title",
   text = "Some text",
   str = `<div class="message">
<h2>${title}</h2>
<article>${text}</article>
</div>`;
console.log( str );

The output is as follows:

<div class="message">
<h2>Some title</h2>
<article>Some text</article>
</div>

If you wonder when can you safely use this syntax, I have a good news for you—this feature is already supported by (almost) all the major script agents (http://kangax.github.io/compat-table/es6/).

Multi-line strings via transpilers

With the advance of ReactJS, Facebook's EcmaScript language extension named JSX (https://facebook.github.io/jsx/) is now really gaining momentum. Apparently influenced by previously mentioned E4X, they proposed a kind of string literal for XML-like content without any screening at all. This type supports template interpolation similar to ES6 templates:

"use strict";
var Hello = React.createClass({
render: function() {
   return <div class="message">
<h2>{this.props.title}</h2>
<article>{this.props.text}</article>
</div>;
}
});

React.render(<Hello title="Some title" text="Some text" />, node);

Another way to declare multiline strings is by using CommonJS Compiler (http://dsheiko.github.io/cjsc/). While resolving the 'require' dependencies, the compiler transforms any content that is not .js/.json content into a single-line string:

foo.txt

Lorem ipsum dolor sit amet,
consectetur adipiscing elit. Nunc ornare,
diam ultricies vehicula aliquam, mauris
ipsum dapibus dolor, quis fringilla leo ligula non neque

consumer.js

var str = require( "./foo.txt" );
console.log( str );

Manipulating arrays in the ES5 way

Some years ago when the support of ES5 features was poor (EcmaScript 5th edition was finalized in 2009), libraries such as Underscore and Lo-Dash got highly popular as they provided a comprehensive set of utilities to deal with arrays/collections. Today, many developers still use third-party libraries (including jQuery/Zepro) for methods such as map, filter, every, some, reduce, and indexOf, while these are available in the native form of JavaScript. It still depends on how you use such libraries, but it may likely happen that you don't need them anymore. Let's see what we have now in JavaScript.

Array methods in ES5

Array.prototype.forEach is probably the most used method of the arrays. That is, it is the native implementation of _.each, or for example, of the $.each utilities. As parameters, forEach expects an iteratee callback function and optionally a context in which you want to execute the callback. It passes to the callback function an element value, an index, and the entire array. The same parameter syntax is used for most array manipulation methods. Note that jQuery's $.each has the inverted callback parameters order:

"use strict";
var data = [ "bar", "foo", "baz", "qux" ];
data.forEach(function( val, inx ){
console.log( val, inx );
});

Array.prototype.map produces a new array by transforming the elements of a given array:

"use strict";
var data = { bar: "bar bar", foo: "foo foo" },
   // convert key-value array into url-encoded string
   urlEncStr = Object.keys( data ).map(function( key ){
     return key + "=" + window.encodeURIComponent( data[ key ] );
   }).join( "&" );

console.log( urlEncStr ); // bar=bar%20bar&foo=foo%20foo

Array.prototype.filter returns an array, which consists of given array values that meet the callback's condition:

"use strict";
var data = [ "bar", "foo", "", 0 ],
   // remove all falsy elements
   filtered = data.filter(function( item ){
     return !!item;
   });

console.log( filtered ); // ["bar", "foo"]

Array.prototype.reduce/Array.prototype.reduceRight retrieves the product of values in an array. The method expects a callback function and optionally the initial value as arguments. The callback function receive four parameters: the accumulative value, current one, index and original array. So we can, for an instance, increment the accumulative value by the current one (return acc += cur;) and, thus, we will get the sum of array values.

Besides calculating with these methods, we can concatenate string values or arrays:

"use strict";
var data = [[ 0, 1 ], [ 2, 3 ], [ 4, 5 ]],
   arr = data.reduce(function( prev, cur ) {
     return prev.concat( cur );
   }),
   arrReverse = data.reduceRight(function( prev, cur ) {
     return prev.concat( cur );
   });

console.log( arr ); // [0, 1, 2, 3, 4, 5]
console.log( arrReverse ); // [4, 5, 2, 3, 0, 1]

Array.prototype.some tests whether any (or some) values of a given array meet the callback condition:

"use strict";
var bar = [ "bar", "baz", "qux" ],
   foo = [ "foo", "baz", "qux" ],
   /**
   * Check if a given context (this) contains the value
   * @param {*} val
   * @return {Boolean}
   */
   compare = function( val ){
     return this.indexOf( val ) !== -1;
   };

console.log( bar.some( compare, foo ) ); // true

In this example, we checked whether any of the bar array values are available in the foo array. For testability, we need to pass a reference of the foo array into the callback. Here we inject it as context. If we need to pass more references, we would push them in a key-value object.

As you probably noticed, we used in this example Array.prototype.indexOf. The method works the same as String.prototype.indexOf. This returns an index of the match found or -1.

Array.prototype.every tests whether every value of a given array meets the callback condition:

"use strict";
var bar = [ "bar", "baz" ],
   foo = [ "bar", "baz", "qux" ],
   /**
   * Check if a given context (this) contains the value
   * @param {*} val
   * @return {Boolean}
   */
   compare = function( val ){
     return this.indexOf( val ) !== -1;
   };

console.log( bar.every( compare, foo ) ); // true

If you are still concerned about support for these methods in a legacy browser as old as IE6-7, you can simply shim them with https://github.com/es-shims/es5-shim.

Array methods in ES6

In ES6, we get just a few new methods that look rather like shortcuts over the existing functionality.

Array.prototype.fill populates an array with a given value, as follows:

"use strict";
var data = Array( 5 );
console.log( data.fill( "bar" ) ); // ["bar", "bar", "bar", "bar", "bar"]

Array.prototype.includes explicitly checks whether a given value exists in the array. Well, it is the same as arr.indexOf( val ) !== -1, as shown here:

"use strict";
var data = [ "bar", "foo", "baz", "qux" ];
console.log( data.includes( "foo" ) );

Array.prototype.find filters out a single value matching the callback condition. Again, it's what we can get with Array.prototype.filter. The only difference is that the filter method returns either an array or a null value. In this case, this returns a single element array, as follows:

"use strict";
var data = [ "bar", "fo", "baz", "qux" ],
   match = function( val ){
     return val.length < 3;
   };
console.log( data.find( match ) ); // fo

Traversing an object in an elegant, reliable, safe, and fast way

It is a common case when we have a key-value object (let's say options) and need to iterate it. There is an academic way to do this, as shown in the following code:

"use strict";
var options = {
   bar: "bar",
   foo: "foo"
   },
   key;
for( key in options ) {
console.log( key, options[ key] );
}

The preceding code outputs the following:

bar bar
foo foo

Now let's imagine that any of the third-party libraries that you load in the document augments the built-in Object:

Object.prototype.baz = "baz";

Now when we run our example code, we will get an extra undesired entry:

bar bar
foo foo
baz baz

The solution to this problem is well known, we have to test the keys with the Object.prototype.hasOwnProperty method:

//…
for( key in options ) {
if ( options.hasOwnProperty( key ) ) {
   console.log( key, options[ key] );
}
}

Iterating the key-value object safely and fast

Let's face the truth—the structure is clumsy and requires optimization (we have to perform the hasOwnProperty test on every given key). Luckily, JavaScript has the Object.keys method that retrieves all string-valued keys of all enumerable own (non-inherited) properties. This gives us the desired keys as an array that we can iterate, for instance, with Array.prototype.forEach:

"use strict";
var options = {
   bar: "bar",
   foo: "foo"
   };
Object.keys( options ).forEach(function( key ){
console.log( key, options[ key] );
});

Besides the elegance, we get a better performance this way. In order to see how much we gain, you can run this online test in distinct browsers such as: http://codepen.io/dsheiko/pen/JdrqXa.

Enumerating an array-like object

Objects such as arguments and nodeList (node.querySelectorAll, document.forms) look like arrays, in fact they are not. Similar to arrays, they have the length property and can be iterated in the for loop. In the form of objects, they can be traversed in the same way that we previously examined. But they do not have any of the array manipulation methods (forEach, map, filter, some and so on). The thing is we can easily convert them into arrays as shown here:

"use strict";
var nodes = document.querySelectorAll( "div" ),
   arr = Array.prototype.slice.call( nodes );

arr.forEach(function(i){
console.log(i);
});

The preceding code can be even shorter:

arr = [].slice.call( nodes )

It's a pretty convenient solution, but looks like a trick. In ES6, we can do the same conversion with a dedicated method:

arr = Array.from( nodes );

The collections of ES6

ES6 introduces a new type of objects—iterable objects. These are the objects whose elements can be retrieved one at a time. They are quite the same as iterators in other languages. Beside arrays, JavaScript received two new iterable data structures, Set and Map. Set which are a collection of unique values:

"use strict";
let foo = new Set();
foo.add( 1 );
foo.add( 1 );
foo.add( 2 );
console.log( Array.from( foo ) ); // [ 1, 2 ]

let foo = new Set(),
   bar = function(){ return "bar"; };
foo.add( bar );
console.log( foo.has( bar ) ); // true

The map is similar to a key-value object, but may have arbitrary values for the keys. And this makes a difference. Imagine that we need to write an element wrapper that provides jQuery-like events API. By using the on method, we can pass not only a handler callback function but also a context (this). We bind the given callback to the cb.bind( context ) context. This means addEventListener receives a function reference different from the callback. How do we unsubscribe the handler then? We can store the new reference in Map by a key composed from an event name and a callback function reference:

"use strict";
/**
* @class
* @param {Node} el
*/
let El = function( el ){
this.el = el;
this.map = new Map();
};
/**
* Subscribe a handler on event
* @param {String} event
* @param {Function} cb
* @param {Object} context
*/
El.prototype.on = function( event, cb, context ){
let handler = cb.bind( context || this );
this.map.set( [ event, cb ], handler );
this.el.addEventListener( event, handler, false );
};
/**
* Unsubscribe a handler on event
* @param {String} event
* @param {Function} cb
*/

El.prototype.off = function( event, cb ){
let handler = cb.bind( context ),
     key = [ event, handler ];
if ( this.map.has( key ) ) {
   this.el.removeEventListener( event, this.map.get( key ) );
   this.map.delete( key );
}
};

Any iterable object has methods, keys, values, and entries, where the keys work the same as Object.keys and the others return array values and an array of key-value pairs respectively. Now let's see how we can traverse the iterable objects:

"use strict";
let map = new Map()
.set( "bar", "bar" )
.set( "foo", "foo" ),
   pair;
for ( pair of map ) {
console.log( pair );
}

// OR
let map = new Map([
   [ "bar", "bar" ],
   [ "foo", "foo" ],
]);
map.forEach(function( value, key ){
console.log( key, value );
});

Iterable objects have manipulation methods such as arrays. So we can use forEach. Besides, they can be iterated by for...in and for...of loops. The first one retrieves indexes and the second, the values.

Summary

This article gives practices and tricks on how to use the JavaScript core features for the maximum effect. This article also discusses the techniques to improve the expressiveness of the code, to master multi-line strings and templating, and to manipulate arrays and array-like objects. Further, we are introduced to the "magic methods" of JavaScript and gives a practical example of their use.

JavaScript was born as a scripting language at the most inappropriate time—the time of browser wars. It was neglected and misunderstood for a decade and endured six editions. And look at it now! JavaScript has become a mainstream programming language.

You can learn more about JavaScript with the help of the following books:

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