4. Core ES6 features
This chapter describes the core ES6 features. These features are easy to adopt; the remaining features are mainly of interest to library authors. I explain each feature via the corresponding ES5 code.
4.1 From var to let/const
In ES5, you declare variables via var. Such variables are function-scoped, their scopes are the innermost enclosing functions. The behavior of var is occasionally confusing. This is an example:var x = 3;
function func(randomize) {
if (randomize) {
var x = Math.random(); // (A) scope: whole function
return x;
}
return x; // accesses the x from line A
}
func(false); // undefined
func() returns undefined may be surprising. You can see why if you rewrite the code so that it more closely reflects what is actually going on:var x = 3;
function func(randomize) {
var x;
if (randomize) {
x = Math.random();
return x;
}
return x;
}
func(false); // undefined
let and const. Such variables are block-scoped, their scopes are the innermost enclosing blocks. let is roughly a block-scoped version of var. const works like let, but creates variables whose values can’t be changed.let and const behave more strictly and
throw more exceptions (e.g. when you access their variables inside their
scope before they are declared). Block-scoping helps with keeping the
effects of code fragments more local (see the next section for a
demonstration). And it’s more mainstream than function-scoping, which
eases moving between JavaScript and other programming languages.If you replace
var with let in the initial version, you get different behavior:let x = 3;
function func(randomize) {
if (randomize) {
let x = Math.random();
return x;
}
return x;
}
func(false); // 3
var with let or const in existing code; you have to be careful during refactoring.My advice is:
- Prefer
const. You can use it for all variables whose values never change. - Otherwise, use
let– for variables whose values do change. - Avoid
var.
4.2 From IIFEs to blocks
In ES5, you had to use a pattern called IIFE (Immediately-Invoked Function Expression) if you wanted to restrict the scope of a variabletmp to a block:(function () { // open IIFE
var tmp = ···;
···
}()); // close IIFE
console.log(tmp); // ReferenceError
let declaration (or a const declaration):{ // open block
let tmp = ···;
···
} // close block
console.log(tmp); // ReferenceError
4.3 From concatenating strings to template literals
With ES6, JavaScript finally gets literals for string interpolation and multi-line strings.4.3.1 String interpolation
In ES5, you put values into strings by concatenating those values and string fragments:function printCoord(x, y) {
console.log('('+x+', '+y+')');
}
function printCoord(x, y) {
console.log(`(${x}, ${y})`);
}
4.3.2 Multi-line strings
Template literals also help with representing multi-line strings.For example, this is what you have to do to represent one in ES5:
var HTML5_SKELETON =
'<!doctype html>\n' +
'<html>\n' +
'<head>\n' +
' <meta charset="UTF-8">\n' +
' <title></title>\n' +
'</head>\n' +
'<body>\n' +
'</body>\n' +
'</html>\n';
var HTML5_SKELETON = '\
<!doctype html>\n\
<html>\n\
<head>\n\
<meta charset="UTF-8">\n\
<title></title>\n\
</head>\n\
<body>\n\
</body>\n\
</html>';
const HTML5_SKELETON = `
<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
</body>
</html>`;
More information: chapter “Template literals and tagged templates”.
4.4 From function expressions to arrow functions
In current ES5 code, you have to be careful withthis whenever you are using function expressions. In the following example, I create the helper variable _this (line A) so that the this of UiComponent can be accessed in line B.function UiComponent() {
var _this = this; // (A)
var button = document.getElementById('myButton');
button.addEventListener('click', function () {
console.log('CLICK');
_this.handleClick(); // (B)
});
}
UiComponent.prototype.handleClick = function () {
···
};
this (line A):function UiComponent() {
var button = document.getElementById('myButton');
button.addEventListener('click', () => {
console.log('CLICK');
this.handleClick(); // (A)
});
}
Arrow functions are especially handy for short callbacks that only return results of expressions.
In ES5, such callbacks are relatively verbose:
var arr = [1, 2, 3];
var squares = arr.map(function (x) { return x * x });
const arr = [1, 2, 3];
const squares = arr.map(x => x * x);
(x) => x * x and x => x * x are both allowed.More information: chapter “Arrow functions”.
4.5 Handling multiple return values
Some functions or methods return multiple values via arrays or objects. In ES5, you always need to create intermediate variables if you want to access those values. In ES6, you can avoid intermediate variables via destructuring.4.5.1 Multiple return values via arrays
exec() returns captured groups via an Array-like object. In ES5, you need an intermediate variable (matchObj in the example below), even if you are only interested in the groups:var matchObj =
/^(\d\d\d\d)-(\d\d)-(\d\d)$/
.exec('2999-12-31');
var year = matchObj[1];
var month = matchObj[2];
var day = matchObj[3];
const [, year, month, day] =
/^(\d\d\d\d)-(\d\d)-(\d\d)$/
.exec('2999-12-31');
4.5.2 Multiple return values via objects
The methodObject.getOwnPropertyDescriptor() returns a property descriptor, an object that holds multiple values in its properties.In ES5, even if you are only interested in the properties of an object, you still need an intermediate variable (
propDesc in the example below):var obj = { foo: 123 };
var propDesc = Object.getOwnPropertyDescriptor(obj, 'foo');
var writable = propDesc.writable;
var configurable = propDesc.configurable;
console.log(writable, configurable); // true true
const obj = { foo: 123 };
const {writable, configurable} =
Object.getOwnPropertyDescriptor(obj, 'foo');
console.log(writable, configurable); // true true
{writable, configurable} is an abbreviation for:{ writable: writable, configurable: configurable }
4.6 From for to forEach() to for-of
Prior to ES5, you iterated over Arrays as follows:var arr = ['a', 'b', 'c'];
for (var i=0; i<arr.length; i++) {
var elem = arr[i];
console.log(elem);
}
forEach():arr.forEach(function (elem) {
console.log(elem);
});
for loop has the advantage that you can break from it, forEach() has the advantage of conciseness.In ES6, the
for-of loop combines both advantages:const arr = ['a', 'b', 'c'];
for (const elem of arr) {
console.log(elem);
}
for-of has got you covered, too, via the new Array method entries() and destructuring:for (const [index, elem] of arr.entries()) {
console.log(index+'. '+elem);
}
for-of loop”.4.7 Handling parameter default values
In ES5, you specify default values for parameters like this:function foo(x, y) {
x = x || 0;
y = y || 0;
···
}
function foo(x=0, y=0) {
···
}
undefined, while it is triggered by any falsy value in the previous ES5 code.More information: section “Parameter default values”.
4.8 Handling named parameters
A common way of naming parameters in JavaScript is via object literals (the so-called options object pattern):selectEntries({ start: 0, end: -1 });
In ES5, you can implement
selectEntries() as follows:function selectEntries(options) {
var start = options.start || 0;
var end = options.end || -1;
var step = options.step || 1;
···
}
function selectEntries({ start=0, end=-1, step=1 }) {
···
}
4.8.1 Making the parameter optional
To make the parameteroptions optional in ES5, you’d add line A to the code:function selectEntries(options) {
options = options || {}; // (A)
var start = options.start || 0;
var end = options.end || -1;
var step = options.step || 1;
···
}
{} as a parameter default value:function selectEntries({ start=0, end=-1, step=1 } = {}) {
···
}
4.9 From arguments to rest parameters
In ES5, if you want a function (or method) to accept an arbitrary number of arguments, you must use the special variable arguments:function logAllArguments() {
for (var i=0; i < arguments.length; i++) {
console.log(arguments[i]);
}
}
args in the example below) via the ... operator:function logAllArguments(...args) {
for (const arg of args) {
console.log(arg);
}
}
function format(pattern, ...args) {
···
}
function format(pattern) {
var args = [].slice.call(arguments, 1);
···
}
More information: section “Rest parameters”.
4.10 From apply() to the spread operator (...)
In ES5, you turn arrays into parameters via apply(). ES6 has the spread operator for this purpose.
4.10.1 Math.max()
Math.max() returns the numerically greatest of its arguments. It works for an arbitrary number of arguments, but not for Arrays.ES5 –
apply():
4.10.2 Array.prototype.push()
Array.prototype.push() appends all of its arguments as
elements to its receiver. There is no method that destructively appends
an Array to another one.ES5 –
apply():var arr1 = ['a', 'b'];
var arr2 = ['c', 'd'];
arr1.push.apply(arr1, arr2);
// arr1 is now ['a', 'b', 'c', 'd']
const arr1 = ['a', 'b'];
const arr2 = ['c', 'd'];
arr1.push(...arr2);
// arr1 is now ['a', 'b', 'c', 'd']
...)”.
4.11 From concat() to the spread operator (...)
The spread operator can also (non-destructively) turn the contents of
its operand into Array elements. That means that it becomes an
alternative to the Array method concat().ES5 –
concat():var arr1 = ['a', 'b'];
var arr2 = ['c'];
var arr3 = ['d', 'e'];
console.log(arr1.concat(arr2, arr3));
// [ 'a', 'b', 'c', 'd', 'e' ]
const arr1 = ['a', 'b'];
const arr2 = ['c'];
const arr3 = ['d', 'e'];
console.log([...arr1, ...arr2, ...arr3]);
// [ 'a', 'b', 'c', 'd', 'e' ]
...)”.4.12 From function expressions in object literals to method definitions
In JavaScript, methods are properties whose values are functions.In ES5 object literals, methods are created like other properties. The property values are provided via function expressions.
var obj = {
foo: function () {
···
},
bar: function () {
this.foo();
}, // trailing comma is legal in ES5
}
const obj = {
foo() {
···
},
bar() {
this.foo();
},
}
4.13 From constructors to classes
ES6 classes are mostly just more convenient syntax for constructor functions.4.13.1 Base classes
In ES5, you implement constructor functions directly:function Person(name) {
this.name = name;
}
Person.prototype.describe = function () {
return 'Person called '+this.name;
};
class Person {
constructor(name) {
this.name = name;
}
describe() {
return 'Person called '+this.name;
}
}
function needed. Also note that there are no commas between the parts of a class.4.13.2 Derived classes
Subclassing is complicated in ES5, especially referring to super-constructors and super-properties. This is the canonical way of creating a sub-constructorEmployee of Person:function Employee(name, title) {
Person.call(this, name); // super(name)
this.title = title;
}
Employee.prototype = Object.create(Person.prototype);
Employee.prototype.constructor = Employee;
Employee.prototype.describe = function () {
return Person.prototype.describe.call(this) // super.describe()
+ ' (' + this.title + ')';
};
extends clause:class Employee extends Person {
constructor(name, title) {
super(name);
this.title = title;
}
describe() {
return super.describe() + ' (' + this.title + ')';
}
}
4.14 From custom error constructors to subclasses of Error
In ES5, it is impossible to subclass the built-in constructor for exceptions, Error. The following code shows a work-around that gives the constructor MyError important features such as a stack trace:function MyError() {
// Use Error as a function
var superInstance = Error.apply(null, arguments);
copyOwnPropertiesFrom(this, superInstance);
}
MyError.prototype = Object.create(Error.prototype);
MyError.prototype.constructor = MyError;
function copyOwnPropertiesFrom(target, source) {
Object.getOwnPropertyNames(source)
.forEach(function(propKey) {
var desc = Object.getOwnPropertyDescriptor(source, propKey);
Object.defineProperty(target, propKey, desc);
});
return target;
};
class MyError extends Error {
}
4.15 From objects to Maps
Using the language construct object as a map from strings to arbitrary values (a data structure) has always been a makeshift solution in JavaScript. The safest way to do so is by creating an object whose prototype isnull. Then you still have to ensure that no key is ever the string '__proto__', because that property key triggers special functionality in many JavaScript engines.The following ES5 code contains the function
countWords that uses the object dict as a map:var dict = Object.create(null);
function countWords(word) {
var escapedWord = escapeKey(word);
if (escapedWord in dict) {
dict[escapedWord]++;
} else {
dict[escapedWord] = 1;
}
}
function escapeKey(key) {
if (key.indexOf('__proto__') === 0) {
return key+'%';
} else {
return key;
}
}
Map and don’t have to escape keys. As a downside, incrementing values inside Maps is less convenient.const map = new Map();
function countWords(word) {
const count = map.get(word) || 0;
map.set(word, count + 1);
}
More information:
- Section “The dict Pattern: Objects Without Prototypes Are Better Maps” in “Speaking JavaScript”
- Chapter “Maps and Sets”
4.16 New string methods
The ECMAScript 6 standard library provides several new methods for strings.From
indexOf to startsWith:if (str.indexOf('x') === 0) {} // ES5
if (str.startsWith('x')) {} // ES6
indexOf to endsWith:function endsWith(str, suffix) { // ES5
var index = str.indexOf(suffix);
return index >= 0
&& index === str.length-suffix.length;
}
str.endsWith(suffix); // ES6
indexOf to includes:if (str.indexOf('x') >= 0) {} // ES5
if (str.includes('x')) {} // ES6
join to repeat (the ES5 way of repeating a string is more of a hack):new Array(3+1).join('#') // ES5
'#'.repeat(3) // ES6
4.17 New Array methods
There are also several new Array methods in ES6.
4.17.1 From Array.prototype.indexOf to Array.prototype.findIndex
The latter can be used to find NaN, which the former can’t detect:const arr = ['a', NaN];
arr.indexOf(NaN); // -1
arr.findIndex(x => Number.isNaN(x)); // 1
Number.isNaN() provides a safe way to detect NaN (because it doesn’t coerce non-numbers to numbers):
4.17.2 From Array.prototype.slice() to Array.from() or the spread operator
In ES5, Array.prototype.slice() was used to convert Array-like objects to Arrays. In ES6, you have Array.from():var arr1 = Array.prototype.slice.call(arguments); // ES5
const arr2 = Array.from(arguments); // ES6
...) to convert it to an Array:const arr1 = [...'abc'];
// ['a', 'b', 'c']
const arr2 = [...new Set().add('a').add('b')];
// ['a', 'b']
4.17.3 From apply() to Array.prototype.fill()
In ES5, you can use apply(), as a hack, to create in Array of arbitrary length that is filled with undefined:// Same as Array(undefined, undefined)
var arr1 = Array.apply(null, new Array(2));
// [undefined, undefined]
fill() is a simpler alternative:const arr2 = new Array(2).fill(undefined);
// [undefined, undefined]
fill() is even more convenient if you want to create an Array that is filled with an arbitrary value:// ES5
var arr3 = Array.apply(null, new Array(2))
.map(function (x) { return 'x' });
// ['x', 'x']
// ES6
const arr4 = new Array(2).fill('x');
// ['x', 'x']
fill() replaces all Array elements with the given value. Holes are treated as if they were elements.More information: Sect. “Creating Arrays filled with values”
4.18 From CommonJS modules to ES6 modules
Even in ES5, module systems based on either AMD syntax or CommonJS syntax have mostly replaced hand-written solutions such as the revealing module pattern.ES6 has built-in support for modules. Alas, no JavaScript engine supports them natively, yet. But tools such as browserify, webpack or jspm let you use ES6 syntax to create modules, making the code you write future-proof.
4.18.1 Multiple exports
4.18.1.1 Multiple exports in CommonJS
In CommonJS, you export multiple entities as follows://------ lib.js ------
var sqrt = Math.sqrt;
function square(x) {
return x * x;
}
function diag(x, y) {
return sqrt(square(x) + square(y));
}
module.exports = {
sqrt: sqrt,
square: square,
diag: diag,
};
//------ main1.js ------
var square = require('lib').square;
var diag = require('lib').diag;
console.log(square(11)); // 121
console.log(diag(4, 3)); // 5
square and diag via it://------ main2.js ------
var lib = require('lib');
console.log(lib.square(11)); // 121
console.log(lib.diag(4, 3)); // 5
4.18.1.2 Multiple exports in ES6
In ES6, multiple exports are called named exports and handled like this://------ lib.js ------
export const sqrt = Math.sqrt;
export function square(x) {
return x * x;
}
export function diag(x, y) {
return sqrt(square(x) + square(y));
}
//------ main1.js ------
import { square, diag } from 'lib';
console.log(square(11)); // 121
console.log(diag(4, 3)); // 5
//------ main2.js ------
import * as lib from 'lib'; // (A)
console.log(lib.square(11)); // 121
console.log(lib.diag(4, 3)); // 5
4.18.2 Single exports
4.18.2.1 Single exports in CommonJS
Node.js extends CommonJS and lets you export single values from modules, viamodule.exports://------ myFunc.js ------
module.exports = function () { ··· };
//------ main1.js ------
var myFunc = require('myFunc');
myFunc();
4.18.2.2 Single exports in ES6
In ES6, the same thing is done via a so-called default export (declared viaexport default)://------ myFunc.js ------
export default function () { ··· } // no semicolon!
//------ main1.js ------
import myFunc from 'myFunc';
myFunc();
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