🔁 JavaScript

for vs forEach vs for...of vs map — Which Loop When?

📅 Jul 4, 2026 ⏱ 3 min read

Five ways to loop, each with a personality. The decision rules:

The lineup

for (let i = 0; i < arr.length; i++)  // need the index / break early / max speed
for (const item of arr)               // clean iteration, break works — DEFAULT
for (const key in obj)                // objects only (includes inherited keys!)
arr.forEach((item, i) => ...)         // callback style; no break; skips holes
const out = arr.map(x => x * 2)       // TRANSFORM — returns a new array

The rules

Producing a new array from each element? map. Just doing side effects? for...of or forEach
Need to break/continue/return? for or for...of — forEach cannot break (throwing to escape is a smell)
Objects: Object.entries(obj) + for...of beats for...in (no prototype surprises)

The async trap (interview classic)

// ❌ forEach ignores await — all fire at once, loop "finishes" instantly
items.forEach(async (x) => await save(x));

// ✅ sequential
for (const x of items) await save(x);

// ✅ parallel, properly awaited
await Promise.all(items.map(x => save(x)));

Full lesson: Loops.

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