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Pure Functions in JavaScript

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  • Difficulty Level : Easy
  • Last Updated : 09 May, 2022

A Pure Function is a function (a block of code) that always returns the same result if the same arguments are passed. It does not depend on any state or data change during a program’s execution. Rather, it only depends on its input arguments.

Also a pure function does not produce any observable side effects such as network requests or data mutation etc.

Let’s see the below JavaScript Function:

function calculateGST( productPrice ) {
    return productPrice * 0.05;

The above function will always return the same result, if we pass the same productPrice. In other words, its output doesn’t get affected by any other values / state changes. So we can call “calculateGST” function a Pure Function.

Now, let’s see one more function below:

var tax = 20;
function calculateGST( productPrice ) {
    return productPrice * (tax / 100) + productPrice;

Pause a second and can you guess whether the above function is Pure or not ?

If you guessed that it is isn’t, you are right! It is not a pure function as the output is dependent on an external variable “tax”. So if the tax value is updated somehow, then we will get a different output though we pass the same productPrice as a parameter to the function.

But here we need to make an important note:

Note: If a pure function calls a pure function, this isn’t a side effect and the calling function is still considered pure. (Example: using Math.max() inside a function)

Below are some side effects (but not limited to) which a function should not produce in order to be considered as a pure function –

  • Making a HTTP request
  • Mutating data
  • Printing to a screen or console
  • DOM Query/Manipulation
  • Math.random()
  • Getting the current time

My Personal Notes arrow_drop_up
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