What's the meaning of "=>" (an arrow formed from equals & greater than) in JavaScript?

Question

I know that the >= operator means more than or equal to, but I've seen => in some source code. What's the meaning of that operator?

Here's the code:

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        return;

    saveAsType = fpParams.saveAsType;
    file = fpParams.file;

    continueSave();
}).then(null, Components.utils.reportError);

Answer 1

What It Is

This is an arrow function. Arrow functions are a short syntax, introduced by ECMAscript 6, that can be used similarly to the way you would use function expressions. In other words, you can often use them in place of expressions like function (foo) {...}. But they have some important differences. For example, they do not bind their own values of this (see below for discussion).

Arrow functions are part of the ECMAscript 6 specification. They are not yet supported in all browsers, but they are partially or fully supported in Node v. 4.0+ and in most modern browsers in use as of 2018. (I’ve included a partial list of supporting browsers below).

You can read more in the Mozilla documentation on arrow functions.

From the Mozilla documentation:

An arrow function expression (also known as fat arrow function) has a shorter syntax compared to function expressions and lexically binds the this value (does not bind its own this, arguments, super, or new.target). Arrow functions are always anonymous. These function expressions are best suited for non-method functions and they can not be used as constructors.

A Note on How this Works in Arrow Functions

One of the most handy features of an arrow function is buried in the text above:

An arrow function... lexically binds the this value (does not bind its own this...)

What this means in simpler terms is that the arrow function retains the this value from its context and does not have its own this. A traditional function may bind its own this value, depending on how it is defined and called. This can require lots of gymnastics like self = this;, etc., to access or manipulate this from one function inside another function. For more info on this topic, see the explanation and examples in the Mozilla documentation.

Example Code

Example (also from the docs):

var a = [
  "We're up all night 'til the sun",
  "We're up all night to get some",
  "We're up all night for good fun",
  "We're up all night to get lucky"
];

// These two assignments are equivalent:

// Old-school:
var a2 = a.map(function(s){ return s.length });

// ECMAscript 6 using arrow functions
var a3 = a.map( s => s.length );

// both a2 and a3 will be equal to [31, 30, 31, 31]

---

Notes on Compatibility

You can use arrow functions in Node, but browser support is spotty.

Browser support for this functionality has improved quite a bit, but it still is not widespread enough for most browser-based usages. As of December 12, 2017, it is supported in current versions of:

• Chrome (v. 45+)
• Firefox (v. 22+)
• Edge (v. 12+)
• Opera (v. 32+)
• Android Browser (v. 47+)
• Opera Mobile (v. 33+)
• Chrome for Android (v. 47+)
• Firefox for Android (v. 44+)
• Safari (v. 10+)
• iOS Safari (v. 10.2+)
• Samsung Internet (v. 5+)
• Baidu Browser (v. 7.12+)

Not supported in:

• IE (through v. 11)
• Opera Mini (through v. 8.0)
• Blackberry Browser (through v. 10)
• IE Mobile (through v. 11)
• UC Browser for Android (through v. 11.4)
• QQ (through v. 1.2)

You can find more (and more current) information at CanIUse.com (no affiliation).

Answer 2

That's known as an Arrow Function, part of the ECMAScript 2015 spec...

var foo = ['a', 'ab', 'abc'];

var bar = foo.map(f => f.length);

console.log(bar); // 1,2,3

Shorter syntax than the previous:

// < ES6:
var foo = ['a', 'ab', 'abc'];

var bar = foo.map(function(f) {
  return f.length;
});
console.log(bar); // 1,2,3

DEMO

The other awesome thing is lexical this... Usually, you'd do something like:

function Foo() {
  this.name = name;
  this.count = 0;
  this.startCounting();
}

Foo.prototype.startCounting = function() {
  var self = this;
  setInterval(function() {
    // this is the Window, not Foo {}, as you might expect
    console.log(this); // [object Window]
    // that's why we reassign this to self before setInterval()
    console.log(self.count);
    self.count++;
  }, 1000)
}

new Foo();

But that could be rewritten with the arrow like this:

function Foo() {
  this.name = name;
  this.count = 0;
  this.startCounting();
}

Foo.prototype.startCounting = function() {
  setInterval(() => {
    console.log(this); // [object Object]
    console.log(this.count); // 1, 2, 3
    this.count++;
  }, 1000)
}

new Foo();

DEMO

MDN
More on Syntax

For more, here's a pretty good answer for when to use arrow functions.

Answer 3

These are Arrow Functions

Also known as Fat Arrow Functions. They're a clean and consise way to write function expressions, e.g. function() {}.

Arrow Functions can remove the need of function, return and {} when defining functions. They are one-liners, similar to Lambda Expressions in Java or Python.

Example with no parameters

const queue = ['Dave', 'Sarah', 'Sharon'];
const nextCustomer = () => queue[0];

console.log(nextCustomer()); // 'Dave'

If multiple statements need to be made within the same Arrow Function, you need to wrap, in this example, queue[0] in curley brackets {}. In this case the return statement cannot be omitted.

Example with 1 parameter

const queue = ['Dave', 'Sarah', 'Sharon'];
const addCustomer = name => {
  queue.push(name);
};

addCustomer('Toby');

console.log(queue); // ['Dave', 'Sarah', 'Sharon', 'Toby']

You can omit {} from the above.

When there is a single parameter, the brackets () around the parameter can be omitted.

Example with multiple parameters

const addNumbers = (x, y) => x + y

console.log(addNumbers(1, 5)); // 6

A useful example

const fruits = [
    { name: 'Apple', price: 2 },
    { name: 'Bananna', price: 3 },
    { name: 'Pear', price: 1 }
];

If we wanted to get the price of every fruit in a single array, in ES5 we could do:

fruits.map(function(fruit) {
    return fruit.price;
}); // [2, 3, 1]

In ES6 with the new Arrow Functions, we can make this more concise:

fruits.map(fruit => fruit.price); // [2, 3, 1]

Additional information on Arrow Functions can be found here.

Answer 4

This would be the "arrow function expression" introduced in ECMAScript 6.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/arrow_functions

For historical purposes (if the wiki page changes later), it is:

An arrow function expression has a shorter syntax compared to function expressions and lexically binds the this value. Arrow functions are always anonymous.

Answer 5

just to add another example of what a lambda can do without using map:

a = 10
b = 2

var mixed = (a,b) => a * b; 
// OR
var mixed = (a,b) => { (any logic); return a * b };

console.log(mixed(a,b)) 
// 20

Answer 6

As others have said, it's a new syntax to create functions.

However, this kind of functions differ from normal ones:

• They bind the this value. As explained by the spec,

  An ArrowFunction does not define local bindings for arguments, super, this, or new.target. Any reference to arguments, super, this, or new.target within an ArrowFunction must resolve to a binding in a lexically enclosing environment. Typically this will be the Function Environment of an immediately enclosing function.

  Even though an ArrowFunction may contain references to super, the function object created in step 4 is not made into a method by performing MakeMethod. An ArrowFunction that references super is always contained within a non-ArrowFunction and the necessary state to implement super is accessible via the scope that is captured by the function object of the ArrowFunction.

• They are non-constructors.

  That means they have no [[Construct]] internal method, and thus can't be instantiated, e.g.

  var f = a => a;
  f(123);  // 123
  new f(); // TypeError: f is not a constructor
  

Answer 7

I've read, this is a symbol of Arrow Functions in ES6

this

var a2 = a.map(function(s){ return s.length });

using Arrow Function can be written as

var a3 = a.map( s => s.length );

MDN Docs

Answer 8

Dissatisfied with the other answers. The top voted answer as of 2019/3/13 is factually wrong.

The short terse version of what => means is it's a shortcut writing a function AND for binding it to the current this

const foo = a => a * 2;

Is effectively a shortcut for

const foo = function(a) { return a * 2; }.bind(this);

You can see all the things that got shortened. We didn't need function, nor return nor .bind(this) nor even braces or parentheses

A slightly longer example of an arrow function might be

const foo = (width, height) => {
  const area = width * height;
  return area;
};

Showing that if we want multiple arguments to the function we need parentheses and if we want write more than a single expression we need braces and an explicit return.

It's important to understand the .bind part and it's a big topic. It has to do with what this means in JavaScript.

ALL functions have an implicit parameter called this. How this is set when calling a function depends on how that function is called.

Take

function foo() { console.log(this); }

If you call it normally

function foo() { console.log(this); }
foo();

this will be the global object.

If you're in strict mode

`use strict`;
function foo() { console.log(this); }
foo();

// or

function foo() {
   `use strict`;
   console.log(this);
 }
foo();

It will be undefined

You can set this directly using call or apply

function foo(msg) { console.log(msg, this); }

const obj1 = {abc: 123}
const obj2 = {def: 456}

foo.call(obj1, 'hello');  // prints Hello {abc: 123}
foo.apply(obj2, ['hi']);  // prints Hi {def: 456}

You can also set this implicitly using the dot operator .

function foo(msg) { console.log(msg, this); }
const obj = {
   abc: 123,
   bar: foo,
}
obj.bar('Hola');  // prints Hola {abc:123, bar: f}

A problem comes up when you want to use a function as a callback or a listener. You make class and want to assign a function as the callback that accesses an instance of the class.

class ShowName {
  constructor(name, elem) {
    this.name = name;
    elem.addEventListener('click', function() {
       console.log(this.name);  // won't work
    }); 
  }
}

The code above will not work because when the element fires the event and calls the function the this value will not be the instance of the class.

One common way to solve that problem is to use .bind

class ShowName {
  constructor(name, elem) {
    this.name = name;
    elem.addEventListener('click', function() {
       console.log(this.name); 
    }.bind(this); // <=========== ADDED! ===========
  }
}

Because the arrow syntax does the same thing we can write

class ShowName {
  constructor(name, elem) {
    this.name = name;
    elem.addEventListener('click',() => {
       console.log(this.name); 
    });
  }
}

bind effectively makes a new function. If bind did not exist you could basically make your own like this

function bind(functionToBind, valueToUseForThis) {
  return function(...args) {
    functionToBind.call(valueToUseForThis, ...args);
  };
}

In older JavaScript without the spread operator it would be

function bind(functionToBind, valueToUseForThis) {
  return function() {
    functionToBind.apply(valueToUseForThis, arguments);
  };
}

Understanding that code requires an understanding of closures but the short version is bind makes a new function that always calls the original function with the this value that was bound to it. Arrow functions do the same thing since they are a shortcut for bind(this)

Answer 9

Adding simple CRUD example with Arrowfunction

 //Arrow Function
 var customers   = [
   {
     name: 'Dave',
     contact:'9192631770'
   },
   {
     name: 'Sarah',
     contact:'9192631770'
   },
   {
     name: 'Akhil',
     contact:'9928462656' 
   }],

// No Param READ
 getFirstCustomer = () => { 
   console.log(this);
   return customers[0];
 };
  console.log("First Customer "+JSON.stringify(getFirstCustomer())); // 'Dave' 

   //1 Param SEARCH
  getNthCustomer = index=>{
    if( index>customers.length)
    {
     return  "No such thing";
   }
   else{
       return customers[index];
     } 
  };
  console.log("Nth Customer is " +JSON.stringify(getNthCustomer(1))); 

   //2params ADD
  addCustomer = (name, contact)=> customers.push({
     'name': name,
     'contact':contact
    });
  addCustomer('Hitesh','8888813275');
  console.log("Added Customer "+JSON.stringify(customers)); 

  //2 param UPDATE
  updateCustomerName = (index, newName)=>{customers[index].name= newName};
  updateCustomerName(customers.length-1,"HiteshSahu");
  console.log("Updated Customer "+JSON.stringify(customers));

  //1 param DELETE
  removeCustomer = (customerToRemove) => customers.pop(customerToRemove);
  removeCustomer(getFirstCustomer());
  console.log("Removed Customer "+JSON.stringify(customers)); 

Answer 10

Arrow functions which is denoted by symbol (=>) helps you to create anonymous functions and methods. That leads to more shorter syntax. For example, below is a simple “Add” function which returns addition of two numbers.

function Add(num1 , num2 ){
return num1 + num2;
}

The above function becomes shorter by using “Arrow” syntax as shown below.

Above code has two parts as shown in the above diagram: -

Input: — This section specifies the input parameters to the anonymous function.

Logic: — This section comes after the symbol “=>”. This section has the logic of the actual function.

Many developers think that arrow function makes your syntax shorter, simpler and thus makes your code readable.

If you believe the above sentence, then let me assure you it’s a myth. If you think for a moment a properly written function with name is much readable than cryptic functions created in one line using an arrow symbol.

The main use of arrow function is to ensure that code runs in the callers context.

See the below code in which have a global variable "context" defined , this global variable is accessed inside a function "SomeOtherMethod" which is called from other method "SomeMethod".

This "SomeMethod" has local "context" variable. Now because "SomeOtherMethod" is called from ""SomeMethod" we expect it to display "local context" , but it displays "global context".

var context = “global context”;

function SomeOtherMethod(){
alert(this.context);
}

function SomeMethod(){
this.context = “local context”;
SomeOtherMethod();
}

var instance = new SomeMethod();

But if replace the call by using Arrow function it will display "local context".

var context = "global context";

    function SomeMethod(){
        this.context = "local context";
        SomeOtherMethod = () => {
            alert(this.context);
        }
        SomeOtherMethod();
    }
    var instance = new SomeMethod();

I would encourage you to read this link ( Arrow function in JavaScript ) which explain all the scenarios of javascript context and in which scenarios the callers context is not respected.

You can also see the demonstration of Arrow function with javascript in this youtube video which demonstrates practically the term Context.

Answer 11

As all of the other answers have already said, it's part of ES2015 arrow function syntax. More specifically, it's not an operator, it's a punctuator token that separates the parameters from the body: ArrowFunction : ArrowParameters => ConciseBody. E.g. (params) => { /* body */ }.

Answer 12

As others have stated, regular (traditional) functions use this from the object that called the function, (e.g. a button that was clicked). Instead, arrow functions use this from the object that defines the function.

Consider two almost identical functions:

regular = function() {
  ' Identical Part Here;
}


arrow = () => {
  ' Identical Part Here;
}

The snippet below demonstrates the fundamental difference between what this represents for each function. The regular function outputs [object HTMLButtonElement] whereas the arrow function outputs [object Window].

<html>
 <button id="btn1">Regular: `this` comes from "this button"</button>
 <br><br>
 <button id="btn2">Arrow: `this` comes from object that defines the function</button>
 <p id="res"/>

 <script>
  regular = function() {
    document.getElementById("res").innerHTML = this;
  }

  arrow = () => {
    document.getElementById("res").innerHTML = this;
  }

  document.getElementById("btn1").addEventListener("click", regular);
  document.getElementById("btn2").addEventListener("click", arrow);
 </script>
</html>

Answer 13

ES6 Arrow functions:

In javascript the => is the symbol of an arrow function expression. A arrow function expression does not have its own this binding and therefore cannot be used as a constructor function. for example:

var words = 'hi from outside object';

let obj = {
  words: 'hi from inside object',
  talk1: () => {console.log(this.words)},
  talk2: function () {console.log(this.words)}
}

obj.talk1();  // doesn't have its own this binding, this === window
obj.talk2();  // does have its own this binding, this is obj

Rules of using arrow functions:

• If there is exactly one argument you can omit the parentheses of the argument.
• If you return an expression and do this on the same line you can omit the {} and the return statement

For example:

let times2 = val => val * 2;  
// It is on the same line and returns an expression therefore the {} are ommited and the expression returns implictly
// there also is only one argument, therefore the parentheses around the argument are omitted

console.log(times2(3));