Array API: array.keys()

Array.prototype.keys returns an iterator for all keys in the array

keys() returns an iterator

const arr = ['a', 'b']; const iterator = arr.keys(); assert.deepEqual(iterator.next(), {value: 0, done: false}); assert.deepEqual(iterator.next(), {value: void 0, done: true});

gets all keys

const arr = ['a', 'b']; const keys = Array.from(arr.keys()); assert.deepEqual(keys, [0, 1, 2]);

empty array contains no keys

const arr = ['empty me']; const keys = [...arr.keys()]; assert.equal(keys.length, 0);

a sparse array without real values has keys though

const arr = [,,]; const keys = [...arr.___()]; assert.deepEqual(keys, [0, 1]);

also includes holes in sparse arrays

const arr = ['a', , 'c']; const keys = arr.keys; assert.deepEqual(keys, [0, 1, 2]);

Required Knowledge

• `const` declaration (Block scope)
• array (Destructuring)
• `Array.from()` (Array API)
• protocol (Iterator)

Related Katas

Array API

• array.sort() basics
• array.sort() can take a compare function
• array.shift() (as introduced in ES3)
• array.push() (as introduced in ES3)
• Array.from()
• Array.of()
• array.fill()
• array.find()
• array.findIndex()
• array.entries()
• array.keys()
• array.values()
• array.includes()
• array.toReversed()

Difficulty Level

INTERMEDIATE

First Published

19 May 2015

Stats

5 tests to solve