Array.prototype.keys()
The keys()
method returns a new array iterator object that contains the keys for each index in the array.
Try it
Syntax
js
keys()
Return value
A new iterable iterator object.
Description
When used on sparse arrays, the keys()
method iterates empty slots as if they have the value undefined
.
The keys()
method is generic. It only expects the this
value to have a length
property and integer-keyed properties.
Examples
Using keys() on sparse arrays
Unlike Object.keys()
, which only includes keys that actually exist in the array, the keys()
iterator doesn't ignore holes representing missing properties.
js
const arr = ["a", , "c"];
const sparseKeys = Object.keys(arr);
const denseKeys = [...arr.keys()];
console.log(sparseKeys); // ['0', '2']
console.log(denseKeys); // [0, 1, 2]
Calling keys() on non-array objects
The keys()
method reads the length
property of this
and then yields all integer indices between 0 and length - 1
. No index access actually happens.
js
const arrayLike = {
length: 3,
};
for (const entry of Array.prototype.keys.call(arrayLike)) {
console.log(entry);
}
// 0
// 1
// 2
Specifications
Specification |
---|
ECMAScript Language Specification # sec-array.prototype.keys |
Browser compatibility
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