A quick reference cheatsheet of what's new in JavaScript for ES2015, ES2016, ES2017, ES2018 and beyond
function fn () {
let x = 0
if (true) {
let x = 1 // only inside this `if`
}
}
const a = 1;
let is the new var. Constants (const) work just like let, but cannot be reassigned. See:
Let and const
const message = `Hello ${name}`;
const str = `
hello
the world
`;
Templates and multiline strings. See: template strings
let bin = 0b1010010;
let oct = 0o755;
const byte = 2 ** 8;
Same as: Math.pow(2, 8)
"hello".repeat(3);
"hello".includes("ll");
"hello".startsWith("he");
"hello".padStart(8); // "hello"
"hello".padEnd(8); // "hello"
"hello".padEnd(8, "!"); // hello!!!
"\u1E9B\u0323".normalize("NFC");
Number.EPSILON;
Number.isInteger(Infinity); // false
Number.isNaN("NaN"); // false
Math.acosh(3); // 1.762747174039086
Math.hypot(3, 4); // 5
Math.imul(Math.pow(2, 32) - 1, Math.pow(2, 32) - 2); // 2
//return a real array
Array.from(document.querySelectorAll("*"));
//similar to new Array(...), but without the special single-argument behavior
Array.of(1, 2, 3);
class Circle extends Shape {
constructor (radius) {
this.radius = radius
}
getArea () {
return Math.PI *2 *this.radius
}
expand(n) {
return super.expand(n) *Math.PI
}
static createFromDiameter(diameter) {
return new Circle(diameter /2)
}
Syntactic sugar for prototypes. See: classes
The javascript default field is public (public), if you need to indicate private, you can use (#)
class Dog {
#name;
constructor(name) {
this.#name = name;
}
printName() {
// Only private fields can be called inside the class
console.log(`Your name is ${this.#name}`);
}
}
const dog = new Dog("putty");
//console.log(this.#name)
//Private identifiers are not allowed outside class bodies.
dog.printName();
class ClassWithPrivate {
static #privateStaticField;
static #privateStaticFieldWithInitializer = 42;
static #privateStaticMethod() {
// …
}
}
new Promise((resolve, reject) => {
if (ok) {
resolve(result);
} else {
reject(error);
}
});
for asynchronous programming. See: Promises
promise
.then((result) => { ··· })
.catch((error) => { ··· })
promise
.then((result) => { ··· })
.catch((error) => { ··· })
.finally(() => {
/*logic independent of success/error */
})
The handler is called when the promise is fulfilled or rejected
Promise.all(···)
Promise.race(···)
Promise.reject(···)
Promise.resolve(···)
async function run () {
const user = await getUser()
const tweets = await getTweets(user)
return [user, tweets]
}
async functions are another way to use functions. See:
Async Function
const [first, last] = ["Nikola", "Tesla"];
let { title, author } = {
title: "The Silkworm",
author: "R. Galbraith",
};
Supports matching arrays and objects. See: Destructuring
const scores = [22, 33];
const [math = 50, sci = 50, arts = 50] = scores;
//Result:
//math === 22, sci === 33, arts === 50
A default value can be assigned when destructuring an array or object
function greet({ name, greeting }) {
console.log(`${greeting}, ${name}!`);
}
greet({ name: "Larry", greeting: "Ahoy" });
Destructuring of objects and arrays can also be done in function parameters
function greet({ name = "Rauno" } = {}) {
console.log(`Hi ${name}!`);
}
greet(); // Hi Rauno!
greet({ name: "Larry" }); // Hi Larry!
function printCoordinates({ left: x, top: y }) {
console.log(`x: ${x}, y: ${y}`);
}
printCoordinates({ left: 25, top: 90 });
This example assigns x to the value of the left key
for (let {title, artist} of songs) {
···
}
Assignment expressions also work in loops
const { id, ...detail } = song;
Use the rest(...) operator to extract some keys individually and the rest of the keys in the object
const options = {
...defaults,
visible: true,
};
const options = Object.assign({}, defaults, { visible: true });
The object spread operator allows you to build new objects from other objects. See: Object Spread
const users = [
...admins,
...editors,
'rstacruz'
]
const users = admins.concat(editors).concat(["rstacruz"]);
The spread operator allows you to build new arrays in the same way. See: Spread operator
function greet(name = "Jerry") {
return `Hello ${name}`;
}
function fn(x, ...y) {
// y is an array
return x * y.length;
}
fn(...[1, 2, 3]);
//same as fn(1, 2, 3)
Default (default), rest, spread (extension). See: function parameters
setTimeout(() => {
···
})
readFile('text.txt', (err, data) => {
...
})
arr.map(n => n*2)
//no curly braces = implicit return
//Same as: arr.map(function (n) { return n*2 })
arr.map(n => ({
result: n*2
}))
//Implicitly returning an object requires parentheses around the object
Like a function, but preserves this. See: Arrow functions
function log(x, y = "World") {
console.log(x, y);
}
log("Hello"); // Hello World
log("Hello", "China"); // Hello China
log("Hello", ""); // Hello
function foo({ x, y = 5 } = {}) {
console.log(x, y);
}
foo(); // undefined 5
function foo() {}
foo.name; // "foo"
function foo(a, b) {}
foo.length; // 2
module.exports = { hello, bye };
same below:
module.exports = {
hello: hello,
bye: bye,
};
const App = {
start() {
console.log("running");
},
};
//Same as: App = { start: function () {···} }
const App = {
get closed () {
return this.status === 'closed'
},
set closed (value) {
this.status = value ? 'closed' : 'open'
}
}
let event = "click";
let handlers = {
[`on${event}`]: true,
};
//Same as: handlers = { 'onclick': true }
const fatherJS = { age: 57, name: "Zhang San" }
Object.values(fatherJS)
//[57, "Zhang San"]
Object.entries(fatherJS)
//[["age", 57], ["name", "Zhang San"]]
import "helpers";
//aka: require('···')
import Express from "express";
//aka: const Express = require('···').default || require('···')
import { indent } from "helpers";
//aka: const indent = require('···').indent
import * as Helpers from "helpers";
//aka: const Helpers = require('···')
import { indentSpaces as indent } from "helpers";
//aka: const indent = require('···').indentSpaces
import is the new require(). See: Module imports
export default function () { ··· }
//aka: module.exports.default = ···
export function mymethod () { ··· }
//aka: module.exports.mymethod = ···
export const pi = 3.14159;
//aka: module.exports.pi = ···
const firstName = "Michael";
const lastName = "Jackson";
const year = 1958;
export { firstName, lastName, year };
export * from "lib/math";
export is the new module.exports. See: Module exports
as keyword renamingimport {
lastName as surname // import rename
} from './profile.js';
function v1() { ... }
function v2() { ... }
export { v1 as default };
//Equivalent to export default v1;
export {
v1 as streamV1, // export rename
v2 as streamV2, // export rename
v2 as streamLatestVersion // export rename
};
button.addEventListener("click", (event) => {
import("./dialogBox.js")
.then((dialogBox) => {
dialogBox.open();
})
.catch((error) => {
/*Error handling */
});
});
ES2020 Proposal introduce import() function
const main = document.querySelector("main");
import(`./modules/${someVariable}.js`)
.then((module) => {
module.loadPageInto(main);
})
.catch((err) => {
main.textContent = err.message;
});
ES2020 Added a meta property import.meta to the import command,
which returns the meta information of the current module
new URL("data.txt", import.meta.url);
In the Node.js environment, import.meta.url always returns a local path, that is, a string of the file:URL protocol,
such as file:/// home/user/foo.js
function* idMaker() {
let id = 0;
while (true) {
yield id++;
}
}
let gen = idMaker();
gen.next().value; // → 0
gen.next().value; // → 1
gen.next().value; // → 2
it's complicated. See: Generators
let fibonacci = {
[Symbol.iterator]() {
let pre = 0,
cur = 1;
return {
next() {
[pre, cur] = [cur, pre + cur];
return { done: false, value: cur };
},
};
},
};
for (var n of fibonacci) {
// truncate sequence at 1000
if (n > 1000) break;
console.log(n);
}
For iterating over generators and arrays. See: For..of iteration
var gen = {};
gen[Symbol.iterator] = function* () {
yield 1;
yield 2;
yield 3;
};
[...gen]; // => [1, 2, 3]
The Generator function is assigned to the Symbol.iterator property, so that the gen object has the Iterator
interface, which can be traversed by the ... operator
function* gen() {
/*some code */
}
var g = gen();
g[Symbol.iterator]() === g; // true
gen is a Generator function, calling it will generate a traverser object g. Its Symbol.iterator property, which
is also an iterator object generation function, returns itself after execution