JavaScript Arrow Functions Expressions

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An arrow function expression is a syntactically compact alternative to a regular function expression, although without its own bindings to the this, arguments, super, or new.target keywords. Arrow function expressions are ill suited as methods, and they cannot be used as constructors.

Syntax
Basic syntax

(param1, param2, …, paramN) => { statements } 
(param1, param2, …, paramN) => expression
// equivalent to: => { return expression; }

// Parentheses are optional when there's only one parameter name:
(singleParam) => { statements }
singleParam => { statements }

// The parameter list for a function with no parameters should be written with a pair of parentheses.
() => { statements }

Advanced syntax

// Parenthesize the body of a function to return an object literal expression:
params => ({foo: bar})

// Rest parameters and default parameters are supported
(param1, param2, ...rest) => { statements }
(param1 = defaultValue1, param2, …, paramN = defaultValueN) => { 
statements }

// Destructuring within the parameter list is also supported
var f = ([a, b] = [1, 2], {x: c} = {x: a + b}) => a + b + c;
f(); // 6

You can create a simple HTML page and add the below JavaScript code in order to see the output, or you can clone my GitHub project.

Below JavaScript code is used to show how Regular function and Arrow functions looks like. Take a look:

console.log('Welcome to Arrow Functions!');

// no parameters

// no parameter, regular function
function hello1() {
    return 'Hello!';
}
console.log('no parameters, regular function ' + hello1());

// no parameter, arrow function
const hello2 = () => {
    return 'Hello!';
}
console.log('no parameters, arrow function ' + hello2());

// no parameter, arrow function single line 
const hello3 = () => 'Hello!';
console.log('no parameters, arrow function single line  ' + hello3());

// one parameter

// one parameter, regular function
function hello4(x){
    return 'one parameter, regular function ' + 'Hello, ' + x;
}
console.log(hello4('milburn'));

// one parameter, arrow function
const hello5 = x => {
   return  'one parameter, arrow function ' + 'Hello, ' + x;
}
console.log(hello5('milburn'));

// one parameter, arrow function single line 
const hello6 = x => 'Hello, ' + x;
console.log('one parameter, arrow function single line ' + hello6('milburn'));

//two parameters

// two parameters, regular function
function hello7(x,y){
    return 'Hello, ' + x + ' ' + y; 
}
console.log('two parameters, regular function ' + hello7('milburn', 'gomes'));

// two parameters, arrow function
const hello8 = (x,y) =>{
    return 'two parameters, arrow function ' + 'Hello, ' + x + ' ' + y;
}
console.log(hello8('milburn', 'gomes'));

// two parameters, arrow function single line 
const hello9 = (x,y) =>  "Hello, " + x + " " + y;
console.log('two parameters, arrow function single line ' + hello9('milburn', 'gomes'));

Description

See also "ES6 In Depth: Arrow functions" on hacks.mozilla.org.

Two factors influenced the introduction of arrow functions: the need for shorter functions and the behavior of the this keyword.

Shorter functions

var elements = [
  'Hydrogen',
  'Helium',
  'Lithium',
  'Beryllium'
];

// This statement returns the array: [8, 6, 7, 9]
elements.map(function(element) {
  return element.length;
});

// The regular function above can be written as the arrow function below
elements.map((element) => {
  return element.length;
}); // [8, 6, 7, 9]

// When there is only one parameter, we can remove the surrounding parentheses
elements.map(element => {
  return element.length;
}); // [8, 6, 7, 9]

// When the only statement in an arrow function is `return`, we can remove `return` and remove
// the surrounding curly brackets
elements.map(element => element.length); // [8, 6, 7, 9]

// In this case, because we only need the length property, we can use destructuring parameter:
// Notice that the `length` corresponds to the property we want to get whereas the
// obviously non-special `lengthFooBArX` is just the name of a variable which can be changed
// to any valid variable name you want
elements.map(({ length: lengthFooBArX }) => lengthFooBArX); // [8, 6, 7, 9]

// This destructuring parameter assignment can also be written as seen below. However, note that in
// this example we are not assigning `length` value to the made up property. Instead, the literal name
// itself of the variable `length` is used as the property we want to retrieve from the object.
elements.map(({ length }) => length); // [8, 6, 7, 9] 

Function body

Arrow functions can have either a "concise body" or the usual "block body".

In a concise body, only an expression is specified, which becomes the implicit return value. In a block body, you must use an explicit return statement.

var func = x => x * x;                  
// concise body syntax, implied "return"

var func = (x, y) => { return x + y; }; 
// with block body, explicit "return" needed

Returning object literals

Keep in mind that returning object literals using the concise body syntax params => {object:literal} will not work as expected.

var func = () => { foo: 1 };
// Calling func() returns undefined!

var func = () => { foo: function() {} };
// SyntaxError: function statement requires a name

This is because the code inside braces ({}) is parsed as a sequence of statements (i.e. foo is treated like a label, not a key in an object literal).

You must wrap the object literal in parentheses:

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

Line breaks

An arrow function cannot contain a line break between its parameters and its arrow.

var func = (a, b, c)
  => 1;
// SyntaxError: expected expression, got '=>'

However, this can be amended by putting the line break after the arrow or using parentheses/braces as seen below to ensure that the code stays pretty and fluffy. You can also put line breaks between arguments.

var func = (a, b, c) =>
  1;

var func = (a, b, c) => (
  1
);

var func = (a, b, c) => {
  return 1
};

var func = (
  a,
  b,
  c
) => 1;

// no SyntaxError thrown

Parsing order

Although the arrow in an arrow function is not an operator, arrow functions have special parsing rules that interact differently with operator precedence compared to regular functions.

let callback;

callback = callback || function() {}; // ok

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

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

More examples

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

(() => 'foobar')(); 
// Returns "foobar"
// (this is an Immediately Invoked Function Expression)

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]

// More concise promise chains
promise.then(a => {
  // ...
}).then(b => {
  // ...
});

// Parameterless arrow functions that are visually easier to parse
setTimeout( () => {
  console.log('I happen sooner');
  setTimeout( () => {
    // deeper code
    console.log('I happen later');
  }, 1);
}, 1);

Browser compatibility

See also

"ES6 In Depth: Arrow functions" on hacks.mozilla.org