Array.prototype.keys() returns an iterator of each index (0, 1, 2, …). It does not yield element values, and it does not mutate the array. Pair it with values() and entries() when you need values or both.
array.keys()
["a", "b", "c"].keys()
→ iterator yielding 0, then 1, then 2
for (const i of arr.keys()) { ... }
Array.from(arr.keys()) → [0, 1, 2]
returns an iterator (not an array)
array unchanged
indexes only — not valuesCall array.keys. Start with this line.
array.keys()| Call | What it does |
|---|---|
array.keys() | Primary signature. |
array.keys(...) | Call with the arguments your case needs. |
Built into JavaScript. Call keys(...) on an array instance — no import required.
array.keys()| Parameter | Description |
|---|---|
| Array | Required. Source of indexes |
| Return Value | Description |
|---|---|
| * | Array Iterator of numbers |
for...ofUse keys() when you care about the index, not just the value.
const langs = ["JS", "Python", "Java"];
for (const i of langs.keys()) {
console.log(i + ": " + langs[i]);
}langs.keys() yields 0, then 1, then 2.langs[i] when you need the element too.Array.fromTurn the iterator into a real array of numbers.
const fruits = ["apple", "orange", "banana"];
const indexes = Array.from(fruits.keys());
console.log(JSON.stringify(indexes)); // [0,1,2]
console.log(indexes.length); // 3.map on the iterator directly.[0, 1, 2].[...fruits.keys()] does the same job.keys vs valuesSee what each iterator yields side by side.
const colors = ["red", "green"];
console.log(JSON.stringify(Array.from(colors.keys()))); // [0,1]
console.log(JSON.stringify(Array.from(colors.values()))); // ["red","green"]
console.log(JSON.stringify(Array.from(colors.entries()))); // [[0,"red"],[1,"green"]]0, 1."red", "green".[index, value].Array.prototype.keys() is part of ES2015 (ES6). It has been available in evergreen browsers and Node.js for years.
Supported in Chrome 38+, Firefox 28+, Safari 8+, Edge 12+, and Node.js 0.12+. Not available in Internet Explorer without a polyfill.
Bottom line: Safe for modern web apps and Node LTS. For IE11, feature-detect with if ("keys" in Array.prototype) or use a classic for loop over indexes.
Find the last position of a value in an array.
11 people found this page helpful