ECMAScript ES6 (ES2015) changes overview

JavaScript, Specifications April 22, 2017

I’ve been playing recently with ReactJS a bit, and was pleasantly surprised when seeing great changes, the JavaScript language has undergone, over the last c.a. 2 years.

This made me realize, that i need to study those changes in more detail, which is how this blog entry came to existence 🙂

According to Wikipedia, “ECMAScript (or ES) is a scripting-language specification, standardized by the European Computer Manufacturers Association. (…) JavaScript is the best-known implementation of ECMAScript since the standard was first published, with other well-known implementations including JScript and ActionScript” (anyone remembering the Flash platform authored by Macromedia?).

In June 2015, sixth edition of ECMAScript (ES6) was introduced, which later changed its name to ECMAScript 2015 (ES2015).

Among the design objectives, that the TC39 (Ecma Technical Committee 39) team defined for the new version of the language, were:

  • Goal 1: Be a better language (for writing: complex applications, libraries (possibly including the DOM) shared by those applications, code generators)
  • Goal 2: Improve interoperation (i.e. adopt de facto standards where possible)
  • Goal 3: Versioning (keep versioning as simple and linear as possible)

Some of the new constructs, that caught my attention:

 

1. let/const vs. var

In ES5, you declare variables via var. Such variables are function-scoped, their scopes are the innermost enclosing functions

In ES6, you can additionally declare variables via 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.

var num = 0;

if (num === 0) {
  let localSpeed = 100;
  var globalSpeed = 200;

  for (let i = 0; i < 0; i++){
    num += (localSpeed + globalSpeed) * 1;
  }

  console.log(typeof i);  // undefined
}

console.log(typeof localSpeed);  // undefined
console.log(typeof num);  // number
console.log(typeof globalSpeed);  // number

General advice by Dr. Axel Rauschmayer (author of Exploring ES6):

  • Prefer const. You can use it for all variables whose values never change.
  • Otherwise, use let – for variables whose values do change.
  • Avoid var.

 

2. IIFEs vs. blocks

In ES5, you had to use a pattern called IIFE (Immediately-Invoked Function Expression) if you wanted to restrict the scope of a variable tmp to a block:

(function () {  // open IIFE
  var tmp = ···;
  ···
}());  // close IIFE

console.log(tmp);  // ReferenceError

In ECMAScript 6, you can simply use a block and a let declaration (or a const declaration):

{  // open block
  let tmp = ···;
  ···
}  // close block

console.log(tmp);  // ReferenceError

 

3. concatenating strings vs. template literals

In ES5, you put values into strings by concatenating those values and string fragments:

function printCoord(x, y) {
  console.log('('+x+', '+y+')');
}

In ES6 you can use string interpolation via template literals:

function printCoord(x, y) {
  console.log(`(${x}, ${y})`);
}

Template literals also help with representing multi-line strings.

 

4. function expressions vs. arrow functions

In ES5, such callbacks are relatively verbose:

var arr = [1, 2, 3];
var squares = arr.map(function (x) { return x * x });

In ES6, arrow functions are much more concise:

const arr = [1, 2, 3];
const squares = arr.map(x => x * x);

 

5. for vs. forEach() vs. 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);
}

In ES5, you have the option of using the Array method forEach():

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

A 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);
}

If you want both index and value of each array element, 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);
}

 

6. Handling multiple return values

A. via arrays

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];

In ES6, destructuring makes this code simpler:

const [, year, month, day] = /^(\d\d\d\d)-(\d\d)-(\d\d)$/.exec('2999-12-31');

(The empty slot at the beginning of the Array pattern skips the Array element at index zero.)

B. via objects

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

In ES6, you can use destructuring:

const obj = { foo: 123 };
const {writable, configurable} = Object.getOwnPropertyDescriptor(obj, 'foo');
console.log(writable, configurable);  // true true

 

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;
  ···
}

ES6 has nicer syntax:

function foo(x=0, y=0) {
  ···
}

 

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 });

Two advantages of this approach are: Code becomes more self-descriptive and it is easier to omit arbitrary parameters.

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;
  ···
}

In ES6, you can use destructuring in parameter definitions and the code becomes simpler:

function selectEntries({ start=0, end=-1, step=1 }) {
  ···
}

 

9. arguments vs. 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]);
  }
}

In ES6, you can declare a rest parameter (args in the example below) via the …operator:

function logAllArguments(...args) {
  for (const arg of args) {
    console.log(arg);
  }
}

Rest parameters are even nicer if you are only interested in trailing parameters:

function format(pattern, ...args) {
  ···
}

Handling this case in ES5 is clumsy:

function format(pattern) {
  var args = [].slice.call(arguments, 1);
  ···
}

 

10. apply() vs. the spread operator (…)

In ES5, you turn arrays into parameters via apply().

ES6 has the spread operator for this purpose.

A. Math.max() example

ES5 – apply():

Math.max.apply(Math, [-1, 5, 11, 3])

ES6 – spread operator:

Math.max(...[-1, 5, 11, 3])

B. Array.prototype.push() example

ES5 – apply():

var arr1 = ['a', 'b'];
var arr2 = ['c', 'd'];

arr1.push.apply(arr1, arr2); // arr1 is now ['a', 'b', 'c', 'd']

ES6 – spread operator:

const arr1 = ['a', 'b'];
const arr2 = ['c', 'd'];

arr1.push(...arr2); // arr1 is now ['a', 'b', 'c', 'd']

 

11. concat() vs. 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' ]

ES6 – spread operator:

const arr1 = ['a', 'b'];
const arr2 = ['c'];
const arr3 = ['d', 'e'];

console.log([...arr1, ...arr2, ...arr3]); // [ 'a', 'b', 'c', 'd', 'e' ]

 

12. function expressions in object literals vs. 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
}

ES6 has method definitions, special syntax for creating methods:

const obj = {
  foo() {
    ···
  },
  bar() {
    this.foo();
  },
}

 

13. constructors vs. classes

ES6 classes are mostly just more convenient syntax for constructor functions.

A. Base classes

In ES5, you implement constructor functions directly:

function Person(name) {
  this.name = name;
}
Person.prototype.describe = function () {
  return 'Person called '+this.name;
};

Note the compact syntax for method definitions – no keyword function needed.

Also note that there are no commas between the parts of a class

B. Derived classes

Subclassing is complicated in ES5, especially referring to super-constructors and super-properties.

This is the canonical way of creating a sub-constructor Employee 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 + ')';
};

ES6 has built-in support for subclassing, via the extends clause:

class Employee extends Person {
  constructor(name, title) {
    super(name);
    this.title = title;
  }
  describe() {
    return super.describe() + ' (' + this.title + ')';
  }
}

 

14. custom error constructors vs. 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() {
  var superInstance = Error.apply(null, arguments); // Use Error as a function
  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;
};

In ES6, all built-in constructors can be subclassed, which is why the following code achieves what the ES5 code can only simulate:

class MyError extends Error {
}

 

15. objects vs. 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 is null. 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 dictas 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;
  }
}

In ES6, you can use the built-in data structure 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);
}

Another benefit of Maps is that you can use arbitrary values as keys, not just strings.

 

16. New string methods

A. indexOf vs. startsWith

if (str.indexOf('x') === 0) {} // ES5
if (str.startsWith('x')) {} // ES6

B. indexOf vs. endsWith

function endsWith(str, suffix) { // ES5
  var index = str.indexOf(suffix);
  return index >= 0 && index === str.length-suffix.length;
}
str.endsWith(suffix); // ES6

C. indexOf vs. includes

if (str.indexOf('x') >= 0) {} // ES5
if (str.includes('x')) {} // ES6

D. join vs. repeat (the ES5 way of repeating a string is more of a hack):

new Array(3+1).join('#') // ES5
'#'.repeat(3) // ES6

 

17. New Array methods

A. Array.prototype.indexOf vs. 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

As an aside, the new Number.isNaN() provides a safe way to detect NaN (because it doesn’t coerce non-numbers to numbers):

isNaN('abc') // true
Number.isNaN('abc') // false

B. Array.prototype.slice() vs. 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

If a value is iterable (as all Array-like DOM data structure are by now), you can also use the spread operator (…) to convert it to an Array:

const arr1 = [...'abc']; // ['a', 'b', 'c']
const arr2 = [...new Set().add('a').add('b')]; // ['a', 'b']

C. apply() vs. 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]

In ES6, 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.

 

18. CommonJS modules vs. 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.

A. Multiple exports in CommonJS

//------ 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

Alternatively, you can import the whole module as an object and access square and diag via it:

//------ main2.js ------
var lib = require('lib');

console.log(lib.square(11)); // 121
console.log(lib.diag(4, 3)); // 5

B. 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

The syntax for importing modules as objects looks as follows (line A):

//------ main2.js ------
import * as lib from 'lib'; // (A)

console.log(lib.square(11)); // 121
console.log(lib.diag(4, 3)); // 5

C. Single exports in CommonJS

Node.js extends CommonJS and lets you export single values from modules, via module.exports:

//------ myFunc.js ------
module.exports = function () { ··· };

//------ main1.js ------
var myFunc = require('myFunc');
myFunc();

D. Single exports in ES6

In ES6, the same thing is done via a so-called default export (declared via export default):

//------ myFunc.js ------
export default function () { ··· } // no semicolon!

//------ main1.js ------
import myFunc from 'myFunc';
myFunc();

 

 

That would be it,

Cheers!

 

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