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Array.prototype.entries()
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
The entries() method of Array instances returns a new array iterator object that contains the key/value pairs for each index in the array.
const array = ["a", "b", "c"];
const iterator = array.entries();
console.log(iterator.next().value);
// Expected output: Array [0, "a"]
console.log(iterator.next().value);
// Expected output: Array [1, "b"]
When used on sparse arrays, the entries() method iterates empty slots as if they have the value undefined.
The entries() method is generic. It only expects the this value to have a length property and integer-keyed properties.
js
const a = ["a", "b", "c"];
for (const [index, element] of a.entries()) {
console.log(index, element);
}
// 0 'a'
// 1 'b'
// 2 'c'
js
const array = ["a", "b", "c"];
const arrayEntries = array.entries();
for (const element of arrayEntries) {
console.log(element);
}
// [0, 'a']
// [1, 'b']
// [2, 'c']
entries() will visit empty slots as if they are undefined.
js
for (const element of [, "a"].entries()) {
console.log(element);
}
// [0, undefined]
// [1, 'a']
The entries() method reads the length property of this and then accesses each property whose key is a nonnegative integer less than length.
js
const arrayLike = {
length: 3,
0: "a",
1: "b",
2: "c",
3: "d", // ignored by entries() since length is 3
};
for (const entry of Array.prototype.entries.call(arrayLike)) {
console.log(entry);
}
// [ 0, 'a' ]
// [ 1, 'b' ]
// [ 2, 'c' ]