Side Effects
forEach runs a callback for every element.
It is best when you need side effects like logging or updating external state.
JavaScript Tutorial
JavaScript forEach()
forEach runs a callback for each element in the array.
It is best for side effects, not for building new arrays.
Why We Need It
Some tasks are about doing something for each item, like logging or updating totals.
forEach expresses that intent clearly.
Syntax
array.forEach((value, index, array) => { ... })Basic Example
1. Basic forEach
const nums = [1, 2, 3];
nums.forEach((n) => console.log(n));Run a callback for each item.
Real World Example
2. Using index
const items = ["a", "b"];
items.forEach((item, i) => console.log(i, item));Access index and value.
Multiple Use Cases
No Return
forEach always returns undefined.
If you need a new array, use map instead.
Cannot Break
You cannot break out of forEach early.
Use a for...of loop when you need break or continue.
More Examples
3. Update external state
const nums = [1, 2, 3];
let total = 0;
nums.forEach((n) => {
total += n;
});
console.log(total);Use forEach for side effects.
4. No early exit
const nums = [1, 2, 3];
nums.forEach((n) => {
if (n === 2) return;
console.log(n);
});return only skips the current callback.
Comparison
Without
const out = [];
for (const n of nums) {
out.push(n * 2);
}With
const out = nums.map((n) => n * 2);Common Mistakes and Fixes
Using forEach to build arrays
Use map or filter to return new arrays.
Trying to break
Use for...of if you need break or continue.
Expecting a return value
forEach always returns undefined.
Interview Questions
Does forEach return anything?
No, it returns undefined.
Can you break out of forEach?
No, use a loop instead.
When is forEach appropriate?
When you need side effects, not transformations.
Practice Problem
Practice: Log each item with its index using forEach.
const items = ["x", "y", "z"];
// TODO: log index and value
One Possible Solution
const items = ["x", "y", "z"];
items.forEach((item, i) => console.log(i, item));Frequently Asked Questions
Does forEach return a new array?
No, it returns undefined.
Can I break out of forEach?
No, use a loop if you need early exit.
When should I use forEach?
For side effects like logging or updating external state.
Try It Yourself
Try changing the array items and see the output.