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How to use the map function in Javascript

Educative Answers Team

Javascript map() creates a new array, which contains the results obtained from iterating over the elements of the specified array and calling the provided function once for each element in order.

Note: map() only works on arrays. It does not execute a function for an empty array.

Syntax

The syntax is as follows:

Description

  • array: the array on which the specified function is called.

  • map: the method called for the array with the required parameters.

  • function(currentValue, index, arr): the function with its parameters which is required to run for each element of the array.

    • currentValue: the value of the current element.

    • index: the index of the current element being processed.

    • arr: the array object on which map() is called.

  • thisValue: value to be used as the function’s this value when executing it. “undefined” will be passed if this value is not provided.

Note: index, arr and thisValue are all optional parameters.

Implementation

Now let’s take a look at an example of an implementation of the map() function.

//creating an array
var my_array = [1, 3, 5, 2, 4];

//map calls a function which has "item" passed as parameter
//map will pass each element of my_array as "item" in this function
//the function will double each element passed to it and return it
result = my_array.map(function(item) {
  return item*2;
});

//prints new list containing the doubled values
console.log(result); 

Note: Since a new array is returned at the end, map() does not change the original array.

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The above example didn’t use any additional parameters.

Let’s look at an example with optional parameters passed into the function:

var my_array = [1,3,5,2,4];

my_array.map(function(item,index,arr) {
  console.log("item: " + item + " at index: " + index + " in the array: " + arr);
});

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