Iterating over Array Indexes

Iterating over Array Indexes with keys()

In JavaScript, the keys() method allows you to create an iterator object that contains the indexes (or keys) of the elements in an array.

The keys() method does not change the original array.

Examples

Example 1, using keys() to iterate over an array:

const fruit = ['watermelon', 'pineapple', 'pear', 'mango']
const fruitKeys = fruit.keys()

for (let key of fruitKeys) {
  console.log(key)
}

// Output:
// 0
// 1
// 2
// 3

Example 2, using keys() with a sparse array:

If an array has “holes” (i.e. missing elements), the keys() method still iterates over all possible indexes.

const animal = ['dog', , 'cat']

for (let key of animal.keys()) {
  console.log(`Animal at index ${key} is ${animal[key]}`)
}

// Output:
// Animal at index 0 is dog
// Animal at index 1 is undefined
// Animal at index 2 is cat