JS: Array.prototype.keys

By Xah Lee. Date: .

New in ES2015.

myArray.keys()

Return a iterator object (also is a iterable object), each entry is the index of the array. That is, 0, 1, 2, etc.

[see JS: Iterator]

[see JS: Iterable]

for (const e of ["a", "b", "c"].keys()) {
 console.log ( e );
}

// prints
// 0
// 1
// 2

A range function:

// a range function
console.log (
 [... (Array(9)).keys()]
);
// [ 0, 1, 2, 3, 4, 5, 6, 7, 8 ]

This example shows the result is iterator and iterable:

const et = ["a", "b", "c"].keys();

// is a iterator
console.log (
 Object.prototype.toString .call (et) === "[object Array Iterator]"
); // true

// is also a iterable
console.log (
    Symbol.iterator in et
); // true

Reference

ECMAScript® 2016 Language Specification#sec-array.prototype.keys

Array Topic

  1. JS: Array Basics
  2. JS: Understand JS Array
  3. JS: Create Array
  4. JS: Sparse Array
  5. JS: Array-Like Object
  6. JS: Array How-To
  7. JS: Array Object
  8. JS: Array.prototype
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