es6-cheatsheet

es6-cheatsheet

A cheatsheet containing ES2015 [ES6] tips, tricks, best practices and code snippet examples for your day to day workflow. Contributions are welcome!

Table of Contents

• var versus let / const
• Replacing IIFEs with Blocks
• Arrow Functions
• Strings
• Destructuring
• Modules
• Parameters
• Classes
• Symbols
• Maps
• WeakMaps
• Promises
• Generators
• Async Await
• Getter/Setter functions
• License

var versus let / const

Besides var, we now have access to two new identifiers for storing values —let and const. Unlike var, let and const statements are not hoisted to the top of their enclosing scope.

An example of using var:

var snack = 'Meow Mix';

function getFood(food) {
    if (food) {
        var snack = 'Friskies';
        return snack;
    }
    return snack;
}

getFood(false); // undefined

However, observe what happens when we replace var using let:

let snack = 'Meow Mix';

function getFood(food) {
    if (food) {
        let snack = 'Friskies';
        return snack;
    }
    return snack;
}

getFood(false); // 'Meow Mix'

This change in behavior highlights that we need to be careful when refactoring legacy code which uses var. Blindly replacing instances of var with let may lead to unexpected behavior.

Note: let and const are block scoped. Therefore, referencing block-scoped identifiers before they are defined will produce a ReferenceError.

console.log(x); // ReferenceError: x is not defined

let x = 'hi';

Best Practice: Leave var declarations inside of legacy code to denote that it needs to be carefully refactored. When working on a new codebase, use let for variables that will change their value over time, and const for variables which cannot be reassigned.

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Replacing IIFEs with Blocks

A common use of Immediately Invoked Function Expressions is to enclose values within its scope. In ES6, we now have the ability to create block-based scopes and therefore are not limited purely to function-based scope.

(function () {
    var food = 'Meow Mix';
}());

console.log(food); // Reference Error

Using ES6 Blocks:

{
    let food = 'Meow Mix';
};

console.log(food); // Reference Error

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Arrow Functions

Often times we have nested functions in which we would like to preserve the context of this from its lexical scope. An example is shown below:

function Person(name) {
    this.name = name;
}

Person.prototype.prefixName = function (arr) {
    return arr.map(function (character) {
        return this.name + character; // Cannot read property 'name' of undefined
    });
};

One common solution to this problem is to store the context of this using a variable:

function Person(name) {
    this.name = name;
}

Person.prototype.prefixName = function (arr) {
    var that = this; // Store the context of this
    return arr.map(function (character) {
        return that.name + character;
    });
};

We can also pass in the proper context of this:

function Person(name) {
    this.name = name;
}

Person.prototype.prefixName = function (arr) {
    return arr.map(function (character) {
        return this.name + character;
    }, this);
};

As well as bind the context:

function Person(name) {
    this.name = name;
}

Person.prototype.prefixName = function (arr) {
    return arr.map(function (character) {
        return this.name + character;
    }.bind(this));
};

Using Arrow Functions, the lexical value of this isn't shadowed and we can re-write the above as shown:

function Person(name) {
    this.name = name;
}

Person.prototype.prefixName = function (arr) {
    return arr.map(character => this.name + character);
};

Best Practice: Use Arrow Functions whenever you need to preserve the lexical value of this.

Arrow Functions are also more concise when used in function expressions which simply return a value:

var squares = arr.map(function (x) { return x * x }); // Function Expression

const arr = [1, 2, 3, 4, 5];
const squares = arr.map(x => x * x); // Arrow Function for terser implementation

Best Practice: Use Arrow Functions in place of function expressions when possible.

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Strings

With ES6, the standard library has grown immensely. Along with these changes are new methods which can be used on strings, such as .includes() and .repeat().

.includes( )

var string = 'food';
var substring = 'foo';

console.log(string.indexOf(substring) > -1);

Instead of checking for a return value > -1 to denote string containment, we can simply use .includes() which will return a boolean:

const string = 'food';
const substring = 'foo';

console.log(string.includes(substring)); // true

.repeat( )

function repeat(string, count) {
    var strings = [];
    while(strings.length < count) {
        strings.push(string);
    }
    return strings.join('');
}

In ES6, we now have access to a terser implementation:

// String.repeat(numberOfRepetitions)
'meow'.repeat(3); // 'meowmeowmeow'

Template Literals

Using Template Literals, we can now construct strings that have special characters in them without needing to escape them explicitly.

var text = "This string contains \"double quotes\" which are escaped.";

let text = `This string contains "double quotes" which don't need to be escaped anymore.`;

Template Literals also support interpolation, which makes the task of concatenating strings and values:

var name = 'Tiger';
var age = 13;

console.log('My cat is named ' + name + ' and is ' + age + ' years old.');

Much simpler:

const name = 'Tiger';
const age = 13;

console.log(`My cat is named ${name} and is ${age} years old.`);

In ES5, we handled new lines as follows:

var text = (
    'cat\n' +
    'dog\n' +
    'nickelodeon'
);

Or:

var text = [
    'cat',
    'dog',
    'nickelodeon'
].join('\n');

Template Literals will preserve new lines for us without having to explicitly place them in:

let text = ( `cat
dog
nickelodeon`
);

Template Literals can accept expressions, as well:

let today = new Date();
let text = `The time and date is ${today.toLocaleString()}`;

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Destructuring

Destructuring allows us to extract values from arrays and objects (even deeply nested) and store them in variables with a more convenient syntax.

Destructuring Arrays

var arr = [1, 2, 3, 4];
var a = arr[0];
var b = arr[1];
var c = arr[2];
var d = arr[3];

let [a, b, c, d] = [1, 2, 3, 4];

console.log(a); // 1
console.log(b); // 2

Destructuring Objects

var luke = { occupation: 'jedi', father: 'anakin' };
var occupation = luke.occupation; // 'jedi'
var father = luke.father; // 'anakin'

let luke = { occupation: 'jedi', father: 'anakin' };
let {occupation, father} = luke;

console.log(occupation); // 'jedi'
console.log(father); // 'anakin'

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Modules

Prior to ES6, we used libraries such as Browserify to create modules on the client-side, and require in Node.js. With ES6, we can now directly use modules of all types (AMD and CommonJS).

Exporting in CommonJS

module.exports = 1;
module.exports = { foo: 'bar' };
module.exports = ['foo', 'bar'];
module.exports = function bar () {};

Exporting in ES6

With ES6, we have various flavors of exporting. We can perform Named Exports:

export let name = 'David';
export let age  = 25;​​

As well as exporting a list of objects:

function sumTwo(a, b) {
    return a + b;
}

function sumThree(a, b, c) {
    return a + b + c;
}

export { sumTwo, sumThree };

We can also export functions, objects and values (etc.) simply by using the export keyword:

export function sumTwo(a, b) {
    return a + b;
}

export function sumThree(a, b, c) {
    return a + b + c;
}

And lastly, we can export default bindings:

function sumTwo(a, b) {
    return a + b;
}

function sumThree(a, b, c) {
    return a + b + c;
}

let api = {
    sumTwo,
    sumThree
};

export default api;

/* Which is the same as
 * export { api as default };
 */

Best Practices: Always use the export default method at the end of the module. It makes it clear what is being exported, and saves time by having to figure out what name a value was exported as. More so, the common practice in CommonJS modules is to export a single value or object. By sticking to this paradigm, we make our code easily readable and allow ourselves to interpolate between CommonJS and ES6 modules.

Importing in ES6

ES6 provides us with various flavors of importing. We can import an entire file:

import 'underscore';

It is important to note that simply importing an entire file will execute all code at the top level of that file.

Similar to Python, we have named imports:

import { sumTwo, sumThree } from 'math/addition';

We can also rename the named imports:

import {
    sumTwo as addTwoNumbers,
    sumThree as sumThreeNumbers
} from 'math/addition';

In addition, we can import all the things (also called namespace import):

import * as util from 'math/addition';

Lastly, we can import a list of values from a module:

import * as additionUtil from 'math/addition';
const { sumTwo, sumThree } = additionUtil;

Importing from the default binding like this:

import api from 'math/addition';
// Same as: import { default as api } from 'math/addition';

While it is better to keep the exports simple, but we can sometimes mix default import and mixed import if needed. When we are exporting like this:

// foos.js
export { foo as default, foo1, foo2 };

We can import them like the following:

import foo, { foo1, foo2 } from 'foos';

When importing a module exported using commonjs syntax (such as React) we can do:

import React from 'react';
const { Component, PropTypes } = React;

This can also be simplified further, using:

import React, { Component, PropTypes } from 'react';

Note: Values that are exported are bindings, not references. Therefore, changing the binding of a variable in one module will affect the value within the exported module. Avoid changing the public interface of these exported values.

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Parameters

In ES5, we had varying ways to handle functions which needed default values, indefinite arguments, and named parameters. With ES6, we can accomplish all of this and more using more concise syntax.

Default Parameters

function addTwoNumbers(x, y) {
    x = x || 0;
    y = y || 0;
    return x + y;
}

In ES6, we can simply supply default values for parameters in a function:

functio