map returns a new array, forEach returns undefined; 2. map is used for data conversion, forEach is used for side effects; 3. map supports chain calls, forEach does not support; 4. Avoid using map to do side effects operations, you should use forEach, use map to achieve results, and use forEach to do things.
In JavaScript, map and forEach are both array methods used to traverse array elements, but their uses and behavior are essentially different. Which one to choose depends on what you want to do.
1. The return value is different: map returns a new array, forEach does not return
This is the most core difference.
-
mapwill create and return a new array . Each element of the new array is the result of the original array element being processed by the callback function. -
forEachjust performs an operation and does not return a value (orundefined).
const numbers = [1, 2, 3, 4]; // map returns the new array const doubled = numbers.map(x => x * 2); console.log(doubled); // [2, 4, 6, 8] // forEach returns undefined const result = numbers.forEach(x => x * 2); console.log(result); // undefined
? So, if you need to "convert data" and get a new array, use
map.
? Don’t useforEachto try to "build" a new array. Although it can be implemented withpush, it is not intuitive or functional.
2. Different usage scenarios
? Use map : When you want to "convert" array elements
For example, rendering React lists, formatting data, etc.
const names = ['alice', 'bob', 'charlie'];
const greetings = names.map(name => `Hello, ${name}!`);
// ['Hello, alice!', 'Hello, bob!', 'Hello, charlie!'] ? Use forEach : When you want to "execute side effects" operations
For example, printing logs, sending requests, modifying external variables, etc.
let total = 0;
numbers.forEach(num => {
total = num;
});
// total is now 10or:
users.forEach(user => sendEmail(user.email));
forEachis more suitable for "doing something" rather than "output data".
3. Chain call support
-
mapreturns an array, so it can be called chained with other array methods, such as.map().filter().sort(). -
forEachreturnsundefinedand cannot be called in chain.
Numbers .map(x => x * 2) .filter(x => x > 5) .forEach(console.log); // Only the last step is printed with forEach
4. Performance and habits
- From a performance perspective, the difference between the two is extremely small, so you don’t need to worry about it.
- But semantically clear:
- Use
mapto mean "I want a new array". - Use
forEachto mean "I want to do something with each element".
- Use
?? Common error: Use
mapto perform side effects (such as sending requests), but do not use its return value.
// ? Not recommended: map should return a new array, but only side effects are done here users.map(user => sendEmail(user.email)); // ? Recommended: Use forEach to mean "execute action" users.forEach(user => sendEmail(user.email));
Summary comparison table
| characteristic | map
|
forEach
|
|---|---|---|
| Return value | New array | undefined
|
| Whether to change the original array | No (return to new array) | No (unless manually modified) |
| Suitable for scenes | Data conversion, generation of new arrays | Execute side effects (such as printing, sending requests) |
| Support chain calls | ? | ? |
Can you break halfway |
? (No one can break directly) | ? (No one can break directly) |
If you need
break, it is recommended to usefor...oforsome/every.
Basically that's it. Remember one sentence:
Use map to achieve results, and use forEach to do things.
The above is the detailed content of JavaScript map vs forEach. For more information, please follow other related articles on the PHP Chinese website!
Hot AI Tools
Undress AI Tool
Undress images for free
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Clothoff.io
AI clothes remover
Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!
Hot Article
Hot Tools
Notepad++7.3.1
Easy-to-use and free code editor
SublimeText3 Chinese version
Chinese version, very easy to use
Zend Studio 13.0.1
Powerful PHP integrated development environment
Dreamweaver CS6
Visual web development tools
SublimeText3 Mac version
God-level code editing software (SublimeText3)
Hot Topics
Why should you place tags at the bottom of the ?
Jul 02, 2025 am 01:22 AM
PlacingtagsatthebottomofablogpostorwebpageservespracticalpurposesforSEO,userexperience,anddesign.1.IthelpswithSEObyallowingsearchenginestoaccesskeyword-relevanttagswithoutclutteringthemaincontent.2.Itimprovesuserexperiencebykeepingthefocusonthearticl
What is event bubbling and capturing in the DOM?
Jul 02, 2025 am 01:19 AM
Event capture and bubble are two stages of event propagation in DOM. Capture is from the top layer to the target element, and bubble is from the target element to the top layer. 1. Event capture is implemented by setting the useCapture parameter of addEventListener to true; 2. Event bubble is the default behavior, useCapture is set to false or omitted; 3. Event propagation can be used to prevent event propagation; 4. Event bubbling supports event delegation to improve dynamic content processing efficiency; 5. Capture can be used to intercept events in advance, such as logging or error processing. Understanding these two phases helps to accurately control the timing and how JavaScript responds to user operations.
A definitive JS roundup on JavaScript modules: ES Modules vs CommonJS
Jul 02, 2025 am 01:28 AM
The main difference between ES module and CommonJS is the loading method and usage scenario. 1.CommonJS is synchronously loaded, suitable for Node.js server-side environment; 2.ES module is asynchronously loaded, suitable for network environments such as browsers; 3. Syntax, ES module uses import/export and must be located in the top-level scope, while CommonJS uses require/module.exports, which can be called dynamically at runtime; 4.CommonJS is widely used in old versions of Node.js and libraries that rely on it such as Express, while ES modules are suitable for modern front-end frameworks and Node.jsv14; 5. Although it can be mixed, it can easily cause problems.
How does garbage collection work in JavaScript?
Jul 04, 2025 am 12:42 AM
JavaScript's garbage collection mechanism automatically manages memory through a tag-clearing algorithm to reduce the risk of memory leakage. The engine traverses and marks the active object from the root object, and unmarked is treated as garbage and cleared. For example, when the object is no longer referenced (such as setting the variable to null), it will be released in the next round of recycling. Common causes of memory leaks include: ① Uncleared timers or event listeners; ② References to external variables in closures; ③ Global variables continue to hold a large amount of data. The V8 engine optimizes recycling efficiency through strategies such as generational recycling, incremental marking, parallel/concurrent recycling, and reduces the main thread blocking time. During development, unnecessary global references should be avoided and object associations should be promptly decorated to improve performance and stability.
How to make an HTTP request in Node.js?
Jul 13, 2025 am 02:18 AM
There are three common ways to initiate HTTP requests in Node.js: use built-in modules, axios, and node-fetch. 1. Use the built-in http/https module without dependencies, which is suitable for basic scenarios, but requires manual processing of data stitching and error monitoring, such as using https.get() to obtain data or send POST requests through .write(); 2.axios is a third-party library based on Promise. It has concise syntax and powerful functions, supports async/await, automatic JSON conversion, interceptor, etc. It is recommended to simplify asynchronous request operations; 3.node-fetch provides a style similar to browser fetch, based on Promise and simple syntax
var vs let vs const: a quick JS roundup explainer
Jul 02, 2025 am 01:18 AM
The difference between var, let and const is scope, promotion and repeated declarations. 1.var is the function scope, with variable promotion, allowing repeated declarations; 2.let is the block-level scope, with temporary dead zones, and repeated declarations are not allowed; 3.const is also the block-level scope, and must be assigned immediately, and cannot be reassigned, but the internal value of the reference type can be modified. Use const first, use let when changing variables, and avoid using var.
JavaScript Data Types: Primitive vs Reference
Jul 13, 2025 am 02:43 AM
JavaScript data types are divided into primitive types and reference types. Primitive types include string, number, boolean, null, undefined, and symbol. The values are immutable and copies are copied when assigning values, so they do not affect each other; reference types such as objects, arrays and functions store memory addresses, and variables pointing to the same object will affect each other. Typeof and instanceof can be used to determine types, but pay attention to the historical issues of typeofnull. Understanding these two types of differences can help write more stable and reliable code.
How to traverse the DOM tree (e.g., parentNode, children, nextElementSibling)?
Jul 02, 2025 am 12:39 AM
DOM traversal is the basis of web page element operation. Common methods include: 1. Use parentNode to obtain the parent node, and can be chained to find it upward; 2. children return a collection of child elements, accessing the first or end child elements through the index; 3. nextElementSibling obtains the next sibling element, and combines previousElementSibling to realize the same-level navigation. Practical applications such as dynamically modifying structures, interactive effects, etc., such as clicking the button to highlight the next brother node. After mastering these methods, complex operations can be achieved through combination.
