How do you access the index of an element in an array during iteration?

TL;DR

To access the index of an element in an array during iteration, you can use methods like forEach, map, for...of with entries, or a traditional for loop. For example, using forEach:

const array = ['a', 'b', 'c'];
array.forEach((element, index) => {
  console.log(index, element);
});

Using forEach

The forEach method executes a provided function once for each array element. The callback function takes three arguments: the current element, the index of the current element, and the array itself.

const array = ['a', 'b', 'c'];
array.forEach((element, index) => {
  console.log(index, element);
});

Using map

The map method creates a new array populated with the results of calling a provided function on every element in the calling array. The callback function also takes three arguments: the current element, the index of the current element, and the array itself.

const array = ['a', 'b', 'c'];
array.map((element, index) => {
  console.log(index, element);
});

Using for...of with entries

The for...of loop can be combined with the entries method to access both the index and the element.

const array = ['a', 'b', 'c'];
for (const [index, element] of array.entries()) {
  console.log(index, element);
}

Using a traditional for loop

A traditional for loop gives you direct access to the index.

const array = ['a', 'b', 'c'];
for (let index = 0; index < array.length; index++) {
  console.log(index, array[index]);
}

Further reading

• MDN Web Docs: Array.prototype.forEach
• MDN Web Docs: Array.prototype.map
• MDN Web Docs: for...of
• MDN Web Docs: Array.prototype.entries

Exercises

const letters = ['a', 'b', 'c'];
const result = [];

for (const [index, value] of letters.entries()) {
  if (index % 2 === 0) result.push(`${index}:${value}`);
}

console.log(result.join('|'));