map method to iterate over an array in JavaScript

map

Given an array of numbers. Using the map method, extract the square root of each array element and write the result to a new array.

Given an array with strings. Using the map method, add the character '!' to the end of the value of each array element.

Given an array with strings. Using the map method, reverse the characters of each string.

Given the following array:

let arr = ['123', '456', '789'];

Using the map method, transform this array into the following:

let arr = [
	[1, 2, 3],
	[4, 5, 6],
	[7, 8, 9]
];

Element key

The callback function can also take a second parameter - JavaScript will place the key of the array element into it.

Let's look at an example. Let's have an array like this:

let arr = ['a', 'b', 'c', 'd', 'e'];

Let's write at the end of each element its index number in the array:

let arr = ['a', 'b', 'c', 'd', 'e'];

let result = arr.map(function(elem, index) {
	return elem + index;
});

console.log(result); // shows ['a0', 'b1', 'c2', 'd3', 'e4']

Given an array of numbers. Using the map method, write to each element of the array the value of this element multiplied by its index number in the array.

Iterating over multidimensional arrays

The map method can also be used to iterate over multidimensional arrays. Let, for example, given an array like this:

let arr = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];

Let's iterate over this array through map and output its elements to the console:

let arr = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];

let result = arr.map(function(elem) {
	console.log(elem);
});

As a result, console.log will output [1, 2, 3], then [4, 5, 6], then [7, 8, 9].

As you can see, subarrays get into the variable elem. Let's now apply the map method to each subarray and square each of its elements:

let arr = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];

let result = arr.map(function(elem) {
	return elem.map(function(num) {
		return num * num;
	});
});

console.log(result);