Arrow functions in javascript

Submitted by hemanth on Thu, 07/05/2012 - 23:06

CoffeeScript is influencing javascript with fat arrows! As python had influenced it before with list comprehensions.

Well, the grammer looks like :

AssignmentExpression :
    ArrowFunctionExpression
    ...

ArrowFunctionExpression :
    ArrowFormalParameters => [lookahead ∉ { "{" }] AssignmentExpression
    ArrowFormalParameters => Block

ArrowFormalParameters :
    ( FormalParameterList_opt )
    Identifier

Grammer in simple English :

  • => works as a function keyword.

  • (Parameters) are placed before the =>.

  • {} are needed for the function body if there are more than one line or else it's optional.

  • For one liners an explicit return is not needed, for others it's needed.

Some examples from the proposal :

// Empty arrow function returns undefined
let empty = () => {};
 
// Single parameter case needs no parentheses around parameter list
let identity = x => x;
 
// No need for parentheses even for lower-precedence expression body
let square = x => x * x;
 
// Parenthesize the body to return an object literal expression
let key_maker = val => ({key: val});
 
// Statement body needs braces, must use 'return' explicitly if not void
let odds = evens.map(v => v + 1);
 
let fives = [];
nats.forEach(v => { if (v % 5 === 0) fives.push(v); });
 
// ''=>'' has only lexical ''this'', no dynamic ''this''
const obj = {
  method: function () {
    return () => this;
  }
};
assert(obj.method()() === obj);
 
let fake = {steal: obj.method()};
assert(fake.steal() === obj);
 
// But ''function'' still has dynamic ''this'' of course
let real = {borrow: obj.method};
assert(real.borrow()() === real);

Do read arrow function syntax simplified on the ES discussion list!

Making is more easier :

var sum = function(a,b){return a+b;}

Can be re-written with fat arrows as :

let sum = (a,b) => return a+b;

Ok, it's time for me to let sleep = dream => 'zzz' + 'zzzz...'; ;) [ Yes dream is lost! ]

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