Closure Functions in JS - ByteScout

Closure Functions in JS

Closure functions are simply a function within a function. It’s a bit of a mouthful, but closures allow you to write neat and robust code. There are numerous applications for them. For example, you can use them in your loops to get the current index, as shown below.

Closure Functions in JS

With this, we can make a map function that returns an array.

Loops are a fundamental part of programming, and often you’ll need to use the same code inside of them multiple times. This is where closures come in handy. By using a closure function inside your loop, you can ensure that the code is only executed once per loop iteration.

When working with children, this can be quite beneficial. Large data sets or when you need to ensure that your code is only executed when certain conditions are met. For example, if you’re filtering an array for specific values, you can use a closure function only to return those values that meet your criteria. When working with closure functions, it’s important to remember that … can often be used to significant effect.

However, there are a few things you should keep in mind when using them.

First and foremost, closure functions can create variables that persist after the process has finished executing. This can be extremely useful, but it also means that you must be careful about the variables you create and how you use them. If you’re not careful, you may inadvertently create a variable that never gets used or overwritten, leading to memory leaks.

Secondly, because closure functions keep track of their execution context, they can sometimes cause issues with reference values. For example, if you have a closure function that modifies an object outside of its scope, any other references to that object will also see the changes that the closure function made. This can be avoided by using copy-by-value semantics whenever possible.

Lastly, closures can make your code more difficult to read and debug, and this is because they tend to introduce additional layers of complexity into your code. As such, it’s essential to use them judiciously and only when necessary.

Examples of Closure Functions in JavaScript

Closure functions are one of the excellent parts of Javascript. Why? Because they make it easy to write super reusable code. So, let’s get started!

# Base Example

Let’s start with a simple base example, and then we can build it up.

All closure functions take an argument as an expression to store in their private variable. This is the value that will be encapsulated for use in the process. Here is a simple example of a closure function:

var save = function(color) {

Once you create a closure function, you can use it like it’s a normal Javascript function:

var green = save('green');


green(); // returns 'green'

We can call our closure function normal even though it has no explicit return statement. This is helpful when you have a long chain of calls that follow the same pattern.


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