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Call map() on an array with a callback that returns the value you want for each element. It returns a new array of those values, leaving the original array unchanged.
What does map() do in JavaScript?
Array.prototype.map() calls a provided function for each present indexed element in an array, then places each callback return value at the corresponding index in a new array. The original array is not transformed in place. This is the method’s defining behavior in MDN’s reference and the ECMA-262, 16th edition (June 2025).
const numbers = [1, 4, 9];
const doubled = numbers.map((number) => number * 2);
console.log(doubled); // [2, 8, 18]
console.log(numbers); // [1, 4, 9]Use the array returned by map(); if you ignore it, you may be using the wrong method for the job.
How do you write a map() callback?
The basic form is array.map(callbackFn). The callback runs for each present element and receives these arguments, in order:
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element: the current value;index: its position in the source array;array: the source array on whichmap()was called.
For example, to turn an array of objects into an array of names, return the property you want from each object:
const users = [
{ id: 7, name: "Ada" },
{ id: 8, name: "Lin" }
];
const names = users.map((user) => user.name);
console.log(names); // ["Ada", "Lin"]Expression-bodied and block-bodied callbacks
An arrow function with an expression body returns that expression automatically:
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const doubled = numbers.map((number) => number * 2);If you use a block body, include an explicit return:
const doubled = numbers.map((number) => {
return number * 2;
});Without a return statement, the callback produces undefined, so each visited position in the mapped array receives undefined.
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Using the index or source array
The second argument is useful when the output depends on position:
const labels = ["Ada", "Lin"].map((name, index) => `${index + 1}. ${name}`);
// ["1. Ada", "2. Lin"]The third argument is the original array, not the new array being constructed. The callback does not receive the in-progress result.
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What does thisArg do?
The full method form is array.map(callbackFn, thisArg). The optional thisArg supplies the callback’s this value when the callback form uses it. Arrow functions capture this lexically, so thisArg does not change an arrow function’s this. For straightforward transformations, an arrow callback usually avoids this complication.
When should you use map() instead of another array method?
Choose based on the result you need:
| Method or form | Use it when | Typical result |
|---|---|---|
map() |
Each input element should produce a corresponding transformed value. | A new array with one result for each present source element. |
filter() |
You want to keep only elements that satisfy a condition. | A new array containing the selected elements. |
reduce() |
You want to combine elements into an accumulated result. | One accumulated value, such as a number or object. |
forEach() or for...of |
You want to perform actions while iterating and do not need a mapped array. | Side effects rather than a transformed array. |
Using map() only for side effects and discarding its returned array is an anti-pattern; MDN recommends forEach() or for...of for that purpose.
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What common map() mistakes should you avoid?
Forgetting to return a value
A callback’s return value becomes the mapped element. If a block-bodied callback omits return, its result is undefined for every element on which it runs. Check that each branch returns the value you intend.
Confusing a missing array slot with undefined
map() skips empty slots in a sparse array, and the corresponding positions in the result remain empty. A slot explicitly assigned undefined is different: it is present, so the callback runs for it.
Passing a function that treats its second argument as something else
map() always supplies the index as the callback’s second argument. If you pass a function whose optional second parameter expects another value, that index may be mistaken for it. Wrap the call in a callback when needed so only the intended argument is passed.
Does map() work only with arrays?
Ordinary arrays are the clearest place to start, but the method can also be applied to array-like objects that have a length and integer-keyed properties. For array inputs, MDN marks map() as widely available across browsers since July 2015.
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