Array.prototype.values()
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since May 2018.
The values() method of Array instances returns a new array iterator object that iterates the value of each item in the array.
Try it
const array = ["a", "b", "c"];
const iterator = array.values();
for (const value of iterator) {
console.log(value);
}
// Expected output: "a"
// Expected output: "b"
// Expected output: "c"
Syntax
values()
Parameters
None.
Return value
A new iterable iterator object.
Description
Array.prototype.values() is the default implementation of Array.prototype[Symbol.iterator]().
Array.prototype.values === Array.prototype[Symbol.iterator]; // true
When used on sparse arrays, the values() method iterates empty slots as if they have the value undefined.
The values() method is generic. It only expects the this value to have a length property and integer-keyed properties.
Examples
Iteration using for...of loop
Because values() returns an iterable iterator, you can use a for...of loop to iterate it.
const arr = ["a", "b", "c", "d", "e"];
const iterator = arr.values();
for (const letter of iterator) {
console.log(letter);
} // "a" "b" "c" "d" "e"
Iteration using next()
Because the return value is also an iterator, you can directly call its next() method.
const arr = ["a", "b", "c", "d", "e"];
const iterator = arr.values();
iterator.next(); // { value: "a", done: false }
iterator.next(); // { value: "b", done: false }
iterator.next(); // { value: "c", done: false }
iterator.next(); // { value: "d", done: false }
iterator.next(); // { value: "e", done: false }
iterator.next(); // { value: undefined, done: true }
console.log(iterator.next().value); // undefined
Reusing the iterable
Warning: The array iterator object should be a one-time use object. Do not reuse it.
The iterable returned from values() is not reusable. When next().done = true or currentIndex > length, the for...of loop ends, and further iterating it has no effect.
const arr = ["a", "b", "c", "d", "e"];
const values = arr.values();
for (const letter of values) {
console.log(letter);
}
// "a" "b" "c" "d" "e"
for (const letter of values) {
console.log(letter);
}
// undefined
If you use a break statement to end the iteration early, the iterator can resume from the current position when continuing to iterate it.
const arr = ["a", "b", "c", "d", "e"];
const values = arr.values();
for (const letter of values) {
console.log(letter);
if (letter === "b") {
break;
}
}
// "a" "b"
for (const letter of values) {
console.log(letter);
}
// "c" "d" "e"
Mutations during iteration
There are no values stored in the array iterator object returned from values(); instead, it stores the address of the array used in its creation, and reads the currently visited index on each iteration. Therefore, its iteration output depends on the value stored in that index at the time of stepping. If the values in the array changed, the array iterator object's values change too.
const arr = ["a", "b", "c", "d", "e"];
const iterator = arr.values();
console.log(iterator); // Array Iterator { }
console.log(iterator.next().value); // "a"
arr[1] = "n";
console.log(iterator.next().value); // "n"
Unlike iterative methods, the array iterator object does not save the array's length at the time of its creation, but reads it once on each iteration. Therefore, if the array grows during iteration, the iterator will visit the new elements too. This may lead to infinite loops.
const arr = [1, 2, 3];
for (const e of arr) {
arr.push(e * 10);
}
// RangeError: invalid array length
Iterating sparse arrays
values() will visit empty slots as if they are undefined.
for (const element of [, "a"].values()) {
console.log(element);
}
// undefined
// 'a'
Calling values() on non-array objects
The values() method reads the length property of this and then accesses each property whose key is a nonnegative integer less than length.
const arrayLike = {
length: 3,
0: "a",
1: "b",
2: "c",
3: "d", // ignored by values() since length is 3
};
for (const entry of Array.prototype.values.call(arrayLike)) {
console.log(entry);
}
// a
// b
// c
Specifications
| Specification |
|---|
| ECMAScript® 2027 Language Specification # sec-array.prototype.values |