Pure vs Impure Functions

These are the two terms that you always hear in a programming language called Pure and Impure functions.

You know Pure Function is always dependent on arguments and there should not be any side effects.

What hack is Side Effects? ☠️

When you try to use external code in the function or if you are mutating a variable then it creates a Side Effect.

Example:

As you can see in the above snippet, mul the function has a dependency on an outer variable called num. Also, mutating num value, which makes its impure function.

Let’s checkout one more quick example:

So, what do you think about the above snippet? 🤔

Yeah! this is also an impure function 😵‍💫

As you all know that JavaScript is synchronous, using console or any callback or promise/fetch will make the function asynchronous.

Here using the console, which is a Web API makes it an impure function.

Let's checkout other side effects with examples:

DOM Manipulation, any callback or reading/writing files
Updating or Modifying a global variable in pure function
Here also, mutating outer variable which is depending on an external variable.

Let's Understand the Pure and Impure function, as now you have an idea of the side effects

Pure Function

It always returns the same result if the same arguments are passed
It never depends on any state/data/change during the execution of a program
It always returns something
Here, writing test cases will be straightforward

Impure Function

Changing the internal state of any argument which has been passed
It may take effect without returning anything
Writing test cases will be a bit complicated as there may be side effects

Pure and Impure Methods

These are pure methods:

Array.map()
Array.reduce()
Array.filter()
Array.concat()
... (spread syntax, which is mostly used to create copies)

These are impure methods:

Array.splice()
Array.sort()
Date.now()
Math.random()

Bonus Point 🔖

You can clone an external state into a pure function.

Cloning an external state into a pure function does not make the function impure.

Example:

In the above snippet, you are using the ... spread operator in the fullName function. It will create a clone of it and then update it. So, it doesn't depend on the name variable.

Thanks for reading the article!

Hope you found this article useful. If you have any questions, feel free to drop them into the comment section.