Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsUse map() when you want to turn array elements into a new array of results. Use forEach() when you want to do something for each element and do not need a result array. If you need to stop early or await asynchronous work, use a loop or a method suited to that control flow instead.
What is the difference between map() and forEach()?
The key difference is what each method is meant to produce: map() returns an array of callback results, while forEach() performs an action and returns undefined. Both run a callback for each present element in the array; neither calls it for an empty slot.
| Question | map() |
forEach() |
|---|---|---|
| Typical purpose | Transform each visited element into an output value. | Perform an action for each visited element. |
| Method return value | A new array containing the callback results. | undefined; callback return values are discarded. |
| Callback returns nothing | The corresponding output position is undefined, unless the input position was a hole. |
The callback’s return value is discarded. |
| Empty slots in a sparse array | Skipped, with holes retained in the returned array. | Skipped. |
| Stop iteration early? | Not an early-exit API. | No native break. |
| Waits for an async callback? | Do not assume iteration waits for callback promises. | No. It is synchronous and does not wait for promises. |
These are differences in behavior and intent, not evidence that one method is faster. MDN’s documentation does not establish a general performance winner.
When should you use map()?
Use map() when each input element should produce a corresponding output value and you will use the resulting array. For example:
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const prices = [10, 20, 30];
const withTax = prices.map((price) => price * 1.2);
// withTax is [12, 24, 36]Here, each price is transformed into a new value, and withTax holds the new array. The original array is not replaced by the method.
For immutable object transformations, return a new object from the callback so the result array contains transformed objects rather than the original ones.
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When should you use forEach()?
Use forEach() when the operation is an action for each element and you do not need to collect callback results into a new array. For example:
const names = ["Ada", "Grace", "Linus"];
names.forEach((name) => console.log(name));This logs each name. The method itself returns undefined, so it is not useful as a value-producing link in a chain.
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If you are mutating existing objects, forEach() communicates that side-effecting intent more clearly than calling map() and discarding the array it creates. MDN Web Docs describes calling map() without using its returned array as an anti-pattern and recommends forEach() or for...of instead: MDN Web Docs: Array.prototype.map().
Common mistakes and edge cases
Forgetting to return from a map() callback
If a callback reaches its end without returning a value, that visited element contributes undefined to the result. It does not disappear from the array. Use filter() to keep or remove elements based on a condition, or flatMap() when its mapping-and-flattening behavior fits.
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Passing parseInt directly to map()
map() calls its callback with the element and its index, among other arguments. parseInt treats its second argument as a radix, so this can produce an unexpected result:
["1", "2", "3"].map(parseInt);
// [1, NaN, NaN]Pass a callback that supplies the intended radix explicitly, or use Number if its conversion behavior is appropriate:
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["1", "2", "3"].map((text) => parseInt(text, 10));Empty slots are not the same as undefined
Both methods skip empty slots in sparse arrays. A slot containing the value undefined, however, is a present element and is visited. When map() skips a hole, the corresponding position in its output remains a hole.
Trying to stop a forEach() callback with break
You cannot use break to exit a forEach() iteration. Use a for...of or for loop when you need general early termination. If the goal matches their specific behavior, some(), every(), or find() can also stop once their condition is met.
Assuming an async callback is awaited
forEach(async (...) => ...) does not wait for the promises returned by the callback. It is neither a sequential-await mechanism nor a completion barrier. Choose an explicit promise composition pattern for asynchronous work; use a loop when the work must be awaited in sequence.
Quick Recap
A quick decision rule
- Need an array of transformed values? Use
map()and use its returned array. - Need to perform an action for each element without collecting results? Use
forEach(). - Need early exit or explicit control over asynchronous work? Use a loop or a method designed for the condition.
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