JavaScript Array keys()

let alphabets = ["A", "B", "C"];

// returns an Array Iterator object that contains the keys
let iterator = alphabets.keys();

// looping through the Iterator object
for (let key of iterator) {
  console.log(key);
}

// Output:
// 0
// 1
// 2

keys() Syntax

The syntax of the keys() method is:

arr.keys()

Here, arr is an array.

keys() Parameters

The keys() method does not take any parameters.

keys() Return Value

• Returns a new Array iterator object.

Example 1: Using keys() Method

let languages = ["JavaScript", "Java", "C++", "Python"];

// returns an Array Iterator Object that contains keys 
let iterator = languages.keys();

// looping through the iterator object
for (let key of iterator) {
  console.log(key);
}

Output

0
1
2
3

In the above example, we have used the keys() method to find out the key of each element in the languages array.

Here, languages.keys() returns an Array Iterator object whose value is stored in iterator.

And finally, we have looped through iterator that prints the key for each element of language.

Example 2: Using key() Method in Array with Holes

The iterator object doesn't skip holes in the array. It also holds the key for empty slots in the array. For example:

let vehicle = ["car", "bus", , "van", "truck"];

// returns an Array Iterator Object that contains keys
let iterator = vehicle.keys();

// looping through the iterator object
for (let key of iterator) {
  console.log(key);
}

Output

0
1
2
3
4

Recommended Reading: JavaScript Array values()