const materials = ["Hydrogen", "Helium", "Lithium", "Beryllium"];console.log(materials.map((material) => material.length));// Expected output: Array [8, 6, 7, 9]

() => expressionparam => expression(param) => expression(param1, paramN) => expression() => {  statements}param => {  statements}(param1, paramN) => {  statements}

(a, b, ...r) => expression(a = 400, b = 20, c) => expression([a, b] = [10, 20]) => expression({ a, b } = { a: 10, b: 20 }) => expression

async param => expressionasync (param1, param2, ...paramN) => {  statements}

Note:Traditional function expressions and arrow functions have more differences than their syntax. We will introduce their behavior differences in more detail in the next few subsections.

// Traditional anonymous function(function (a) {  return a + 100;});// 1. Remove the word "function" and place arrow between the argument and opening body brace(a) => {  return a + 100;};// 2. Remove the body braces and word "return" — the return is implied.(a) => a + 100;// 3. Remove the parameter parenthesesa => a + 100;

// Traditional anonymous function(function (a, b) {  return a + b + 100;});// Arrow function(a, b) => a + b + 100;const a = 4;const b = 2;// Traditional anonymous function (no parameters)(function () {  return a + b + 100;});// Arrow function (no parameters)() => a + b + 100;

// Traditional anonymous function(function (a, b) {  const chuck = 42;  return a + b + chuck;});// Arrow function(a, b) => {  const chuck = 42;  return a + b + chuck;};

// Traditional Functionfunction bob(a) {  return a + 100;}// Arrow Functionconst bob2 = (a) => a + 100;

const func = (x) => x * x;// expression body syntax, implied "return"const func2 = (x, y) => {  return x + y;};// with block body, explicit "return" needed

const func = () => { foo: 1 };// Calling func() returns undefined!const func2 = () => { foo: function () {} };// SyntaxError: function statement requires a nameconst func3 = () => { foo() {} };// SyntaxError: Unexpected token '{'

const func = () => ({ foo: 1 });

"use strict";const obj = {  i: 10,  b: () => console.log(this.i, this),  c() {    console.log(this.i, this);  },};obj.b(); // logs undefined, Window { /* … */ } (or the global object)obj.c(); // logs 10, Object { /* … */ }

"use strict";const obj = {  a: 10,};Object.defineProperty(obj, "b", {  get: () => {    console.log(this.a, typeof this.a, this); // undefined 'undefined' Window { /* … */ } (or the global object)    return this.a + 10; // represents global object 'Window', therefore 'this.a' returns 'undefined'  },});

class C {  a = 1;  autoBoundMethod = () => {    console.log(this.a);  };}const c = new C();c.autoBoundMethod(); // 1const { autoBoundMethod } = c;autoBoundMethod(); // 1// If it were a normal method, it should be undefined in this case

class C {  a = 1;  constructor() {    this.method = this.method.bind(this);  }  method() {    console.log(this.a);  }}

Note:Class fields are defined on theinstance, not on theprototype, so every instance creation would create a new function reference and allocate a new closure, potentially leading to more memory usage than a normal unbound method.

function foo(n) {  const f = () => arguments[0] + n; // foo's implicit arguments binding. arguments[0] is n  return f();}foo(3); // 3 + 3 = 6

function foo(n) {  const f = (...args) => args[0] + n;  return f(10);}foo(1); // 11

const Foo = () => {};const foo = new Foo(); // TypeError: Foo is not a constructorconsole.log("prototype" in Foo); // false

const func = (a, b, c)  => 1;// SyntaxError: Unexpected token '=>'

const func = (a, b, c) =>  1;const func2 = (a, b, c) => (  1);const func3 = (a, b, c) => {  return 1;};const func4 = (  a,  b,  c,) => 1;

let callback;callback = callback || () => {};// SyntaxError: invalid arrow-function arguments

callback = callback || (() => {});

// An empty arrow function returns undefinedconst empty = () => {};(() => "foobar")();// Returns "foobar"// (this is an Immediately Invoked Function Expression)const simple = (a) => (a > 15 ? 15 : a);simple(16); // 15simple(10); // 10const max = (a, b) => (a > b ? a : b);// Easy array filtering, mapping, etc.const arr = [5, 6, 13, 0, 1, 18, 23];const sum = arr.reduce((a, b) => a + b);// 66const even = arr.filter((v) => v % 2 === 0);// [6, 0, 18]const double = arr.map((v) => v * 2);// [10, 12, 26, 0, 2, 36, 46]// More concise promise chainspromise  .then((a) => {    // …  })  .then((b) => {    // …  });// Arrow functions without parameterssetTimeout(() => {  console.log("I happen sooner");  setTimeout(() => {    // deeper code    console.log("I happen later");  }, 1);}, 1);

const obj = {  num: 100,};// Setting "num" on globalThis to show how it is NOT used.globalThis.num = 42;// A traditional function to operate on "this"function add(a, b, c) {  return this.num + a + b + c;}console.log(add.call(obj, 1, 2, 3)); // 106console.log(add.apply(obj, [1, 2, 3])); // 106const boundAdd = add.bind(obj);console.log(boundAdd(1, 2, 3)); // 106

const obj = {  num: 100,};// Setting "num" on globalThis to show how it gets picked up.globalThis.num = 42;// Arrow functionconst add = (a, b, c) => this.num + a + b + c;console.log(add.call(obj, 1, 2, 3)); // 48console.log(add.apply(obj, [1, 2, 3])); // 48const boundAdd = add.bind(obj);console.log(boundAdd(1, 2, 3)); // 48

const obj = {  count: 10,  doSomethingLater() {    setTimeout(function () {      // the function executes on the window scope      this.count++;      console.log(this.count);    }, 300);  },};obj.doSomethingLater(); // logs "NaN", because the property "count" is not in the window scope.

const obj = {  count: 10,  doSomethingLater() {    // The method syntax binds "this" to the "obj" context.    setTimeout(() => {      // Since the arrow function doesn't have its own binding and      // setTimeout (as a function call) doesn't create a binding      // itself, the "obj" context of the outer method is used.      this.count++;      console.log(this.count);    }, 300);  },};obj.doSomethingLater(); // logs 11