JavaScript Array Iteration Methods (for loop, for...in, for...of, forEach, map, filter, every, some)

JavaScript Array Iteration Methods

1. for loop

The most basic — supports break / continue:

const arr = [1, 2, 3];
for (let i = 0; i < arr.length; i++) {
  console.log(arr[i]);
}

2. for...in

Iterates over enumerable property keys — not recommended for arrays (may include prototype properties):

for (const index in arr) {
  console.log(index); // '0', '1', '2' (strings)
}

3. for...of

Iterates over iterable values — great for arrays, supports break:

for (const value of arr) {
  console.log(value); // 1, 2, 3
}

4. forEach

Executes a function for each element — no return value, cannot break:

arr.forEach((value, index) => {
  console.log(index, value);
});

5. map

Returns a new array with each element transformed:

const doubled = arr.map(x => x * 2); // [2, 4, 6]

6. filter

Returns a new filtered array:

const evens = arr.filter(x => x % 2 === 0); // [2]

7. every

Returns true if all elements pass the test:

arr.every(x => x > 0); // true
arr.every(x => x > 1); // false

8. some

Returns true if at least one element passes the test:

arr.some(x => x > 2); // true
arr.some(x => x > 5); // false

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Comparison

Method	Returns	Can break	Use case
for	—	✅	General iteration
for...of	—	✅	Value iteration
forEach	undefined	❌	Side effects
map	New array	❌	Transform elements
filter	New array	❌	Filter elements
every	boolean	❌	All pass?
some	boolean	❌	Any pass?