Arrow functions

An arrow function expression (also known as fat arrow function) has a shorter syntax compared to function expressions and lexically binds the this value. Arrow functions are always anonymous.

Syntax

([param] [, param]) => {
   statements
}

param => expression

Detailed syntax examples can be seen here.

param

The name of an argument. Zero arguments need to be indicated with (). For only one argument the parentheses are not required. (like foo => 1)

statements or expression

Multiple statements need to be enclosed in braces, {}. A single expression, however, requires no braces. The expression is also the implicit return value of that function.

Description

Two factors influenced the introduction of arrow functions: shorter functions and lexical this.

Shorter functions

In some functional patterns, shorter functions are welcome. Compare:

var a = [
  "Hydrogen",
  "Helium",
  "Lithium",
  "Beryl­lium"
];

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

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

Lexical this

Until arrow functions, every new function defined its own this value (a new object in case of a constructor, undefined in strict mode function calls, the context object if the function is called as an "object method", etc.). This proved to be annoying with an object-oriented style of programming.

function Person() {
  // The Person() constructor defines `this` as itself.
  this.age = 0;

  setInterval(function growUp() {
    // In nonstrict mode, the growUp() function defines `this` 
    // as the global object, which is different from the `this`
    // defined by the Person() constructor.
    this.age++;
  }, 1000);
}

var p = new Person();

In ECMAScript 3/5, this issue was fixed by assigning the value in this to a variable that could be closed over.

function Person() {
  var self = this; // Some choose `that` instead of `self`. 
                   // Choose one and be consistent.
  self.age = 0;

  setInterval(function growUp() {
    // The callback refers to the `self` variable of which
    // the value is the expected object.
    self.age++;
  }, 1000);
}

Alternatively, a bound function could be created so that the proper this value would be passed to the growUp function.

Arrow functions capture the this value of the enclosing context, so the following code works as expected.

function Person(){
  this.age = 0;

  setInterval(() => {
    this.age++; // |this| properly refers to the person object
  }, 1000);
}

var p = new Person();

Relation with strict mode

Given that this is lexical, strict mode rules with regard to this are just ignored.

var f = () => {'use strict'; return this};
f() === window; // or the global object

The rest of strict mode rules apply normally.

Invoked through call or apply

Since this is already bound lexically, invoke arrow function through the call or the apply method can only pass in arguments but has no effect on this:

var adder = {
    
        base : 1,
    
        add : function(a) {
            
            var f = v => v + this.base;
            
            return f(a);
        },
        
        addThruCall: function(a) {
            
            var f = v => v + this.base;
            
            var b = {
                    base : 2
                };
            
            return f.call(b, a);
        }
    };

console.log(adder.add(1));         // This would log to 2
console.log(adder.addThruCall(1)); // This would log to 2 still

Examples

// An empty arrow function returns undefined
let empty = () => {};

(() => "foobar")() // returns "foobar" 

var simple = a => a > 15 ? 15 : a; 
simple(16); // 15
simple(10); // 10

let max = (a, b) => a > b ? a : b;

// Easy array filtering, mapping, ...

var arr = [5, 6, 13, 0, 1, 18, 23];
var sum = arr.reduce((a, b) => a + b);  // 66
var even = arr.filter(v => v % 2 == 0); // [6, 0, 18]
var double = arr.map(v => v * 2);       // [10, 12, 26, 0, 2, 36, 46]

Specifications

Browser compatibility

Firefox-specific notes

• The initial implementation of arrow functions in Firefox made them automatically strict. This has been changed as of Firefox 24. The use of "use strict"; is now required.
• Arrow functions are semantically different from the non-standard Expression Closures added in Firefox 3 (details: Javascript 1.8), for Expression Closures do not bind this lexically.
• Prior to Firefox 39, a line terminator (\n) was incorrectly allowed after arrow function arguments. This has been fixed to conform to the ES6 specification and code like () \n => {} will now throw a SyntaxError in this and later versions.