Arrow Functions in ES6

Arrow functions are available to many other modern languages and was one of the features I sorely missed a few years ago when I moved from C# to JavaScript. Fortunately, they’re now part of ES6 and thus available to us in JavaScript. The syntax is quite expressive. We already had anonymous functions, but sometimes it’s nice to have a terse alternative.
Here’s how the syntax looks like if we have a single argument and just want to return the results for an expression.

[1, 2, 3].map(num => num * 2)
// <- [2, 4, 6]

The ES5 equivalent would be as below.

[1, 2, 3].map(function (num) { return num * 2 })
// <- [2, 4, 6]

If we need to declare more arguments (or no arguments), we’ll have to use parenthesis.

[1, 2, 3, 4].map((num, index) => num * 2 + index)
// <- [2, 5, 8, 11]

A cool aspect of arrow functions in ES6 is that they’re bound to their lexical scope. That means that you can say goodbye to var self = this and similar hacks – such as using .bind(this) to preserve the context from within deeply nested methods.

function Timer () {
    this.seconds = 0
    setInterval(() => this.seconds++, 1000)
var timer = new Timer()
setTimeout(() => console.log(timer.seconds), 3100)
// <- 3
  • Terse way to declare a function like param => returnValue
  • Useful when doing functional stuff like [1, 2].map(x => x * 2)
  • Several flavors are available, might take you some getting used to
    • p1 => expr is okay for a single parameter
    • p1 => expr has an implicit return statement for the provided expr expression
    • To return an object implicitly, wrap it in parenthesis () => ({ foo: 'bar' }) or you’ll get an error
    • Parenthesis are demanded when you have zero, two, or more parameters, () => expr or (p1, p2) => expr
    • Brackets in the right-hand side represent a code block that can have multiple statements, () => {}
    • When using a code block, there’s no implicit return, you’ll have to provide it — () => { return 'foo' }
  • You can’t name arrow functions statically, but runtimes are now much better at inferring names for most methods
  • Arrow functions are bound to their lexical scope
    • this is the same this context as in the parent scope
    • this can’t be modified with .call, .apply, or similar “reflection”-type methods
    • arguments is also lexically scoped to the nearest normal function; use (...args) for local arguments
  • Read ES6 Arrow Functions in Depth

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