What is a JavaScript Map?

To get a new array with modified values, we may need to apply a function or operation to each element of an array. This can be accomplished by using a for() or forEach() loop to iterate over the array.

However, JavaScript has a built-in function called array map() that can let you do these kinds of actions elegantly without having to manually iterate through the array.

You will be able to accomplish these actions quickly while keeping your code sane and avoiding the use of large loops by using the array map function. However, if you’re new to javascript programming, the map() function can be a little perplexing.

The Array.prototype property was announced in ECMAScript 5 (ES5) and all modern browsers support it. The Array map() is a method that’s also included in it.  

We had to use objects before ES6 to hold the key value pair. Except for an array, we never had any other data structure.

We now have an alternative way to handle the data, thanks to the arrival of javascript Map and Set.

What is a JavaScript Map?

Map is an iteration method that, like other iteration methods such as forEach(), filter(), reduce(), and others, allows us to loop across a dataset, performing the specified actions on each element.

The map() method produces a new array and populates it with the value obtained by applying the supplied function to each element of the old array. The map function is a non-mutating method, which means it does not affect the original array elements. Instead, it generates a new array and stores the value in it.

Creating Map() in JavaScript

Make sure you have the node js runtime installed so you can run javascript applications in the terminal before you begin. You can use any online compiler if you don’t have a node js runtime installed. There are a plethora of online compilers to choose from.

You may also use the chrome developer tools console to run programs if you’re using Chrome. The chrome developer tools console can be accessed by pressing Ctrl+shift+J (Windows) or Ctrl+Option+J (Mac).

Prior to the release of javascript Map, developers had a workaround for dealing with key value pairs, however, that workaround had limits. Let’s look at the workaround first to understand why the Maps were required.

Map() JavaScript

This strategy works well, however, the problem becomes more complicated when we encounter the object properties’ limits.

Map JavaScript Example

To save values, Object turns everything to a string internally. Hence, the number 5 is changed to “5”. As a result, we are unable to use the numeric key.

When we make use of objects as the key, the same thing happens.

Maps are an ordered set of key-value pairs with any type of key and value.


Map() JavaScript

If an iterable object is supplied with key-value pairs in its elements, such as Array(arrays with two elements, e.g. [[1, ‘one’],[2, ‘two’]], then all of its items will be added to the Map.

Undefined values are handled as null values.

Map() JavaScript examples

Objects can also be used as a key.

JavaScript Map Object

Why Using Map() Has Greater Benefits As Compared To For Loop

The following are some of the benefits of using a map rather than a loop:

  • The map() function is immutable, which means your original array will be preserved. If you require your original array somewhere else, this is really beneficial.
  • Cleaner code: When writing code, the map function helps you cut down on unneeded lines, making your code cleaner and more readable.
  • Quicker coding: Compared to traditional loops, the map function is faster to code.

When Not To Use Map()?

  • If you’re not going to use the array returned by the map method, don’t use it.
  • From the callback function, you don’t return anything.

In both of these circumstances, you should avoid using the map function. In such cases, a for() or forEach() loop should be used instead.

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