Programming Tutorials

(追記) (追記ここまで)

map() in JavaScript

By: Urmila in Javascript Tutorials on 2023年04月26日 [フレーム]

In JavaScript, map() is a built-in method available on arrays that allows you to iterate through each element of an array and create a new array by performing some operation on each element. The syntax of map() is as follows:

array.map(function(currentValue, index, arr), thisValue)

Here, array is the array you want to operate on, and the function is what you want to do with each element of the array.

The function takes in three parameters:

currentValue: the current element being processed in the array
index: the index of the current element being processed
arr: the original array that map() was called on

The thisValue parameter is optional and is used to set the value of this inside the function.

map() returns a new array with the same length as the original array, where each element is the result of the function applied to the corresponding element in the original array.

Here's an example that uses map() to create a new array of numbers that are double the original numbers:

const numbers = [1, 2, 3, 4, 5];
const doubledNumbers = numbers.map(function(num) {
 return num * 2;
});
console.log(doubledNumbers); // [2, 4, 6, 8, 10]

In this example, we're using an anonymous function that takes in a num parameter and returns num * 2. The map() method iterates through each element in the numbers array, calls the function with the current element as the num parameter, and creates a new array doubledNumbers with the doubled values. Note that we are only passing num a single parameter which in this case is the currentValue instead of three parameters currentValue, index, arr. So unless you need to use them in the function, you don't have to pass it.