Reflect: Reflect.apply()

Reflect.apply calls a target function

it is a static method

const actualType = '???'; assert.equal(actualType, typeof Reflect.apply)

the 1st parameter

is a callable, e.g. a function

let fn; assert.equal(Reflect.apply(fn, void 0, []), 42);

passing it a non-callable throws a TypeError

const applyOnUncallable = () => Reflect.apply(() => {}, void 0, []); assert.throws(applyOnUncallable, TypeError);

the 2nd parameter

is the scope (or the this)

class FourtyTwo { constructor() { this.value = 42} fn() {return this.value} } let instance = new FourtyTwo(); const fourtyTwo = Reflect.apply(instance.fn, ___, []); assert.deepEqual(fourtyTwo, 42);

the 3rd parameter

must be an array (or array-like)

const thirdParam = 'should be array-like'; assert.doesNotThrow(() => Reflect.apply(() => void 0, null, thirdParam));

is an array of parameters passed to the call

let emptyArrayWithFiveElements = Reflect.apply(Array); assert.deepEqual(emptyArrayWithFiveElements.fill(42), [42, 42, 42, 42, 42]);

example usages

simple function call

const fn = () => ':('; assert.equal(Reflect.apply(fn, void 0, []), 'the return value');

call a function on an array

const fn = [].join; assert.deepEqual(Reflect.apply(fn, [0, 23, 42], [1]), [23, 42]);

pass in the this that the function to call needs

class Bob { constructor() { this._name = 'Bob'; } name() { return this._name; } } const bob = new Bob(); const scope = Bob; assert.equal(Reflect.apply(bob.name, scope, []), 'Bob');

Links

spec How this function is specified.
spec How the 3rd parameter gets processed, as an `CreateListFromArrayLike`.
docs mdn The MDN docs for this function.
announcement Announcement of this kata on twitter.

Required Knowledge

• basics (Arrow functions)
• `let` declaration (Block scope)
• `const` declaration (Block scope)
• creation (Class)
• `array.fill()` (Array API)

Related Katas

Reflect

• basics
• Reflect.apply()
• Reflect.getPrototypeOf()
• Reflect.construct()
• Reflect.defineProperty()

Difficulty Level

INTERMEDIATE

First Published

3 July 2015

Stats

9 tests to solve