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.

let num = 10
function mul(val) {
return num += val

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:

function hello() {
console.log("Hello Folks!");

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:

  1. DOM Manipulation, any callback or reading/writing files

    function mul(a,b) {
    document.write("Started Multiplication")
    return a * b
  2. Updating or Modifying a global variable in pure function

    let x = 10
    function mul(a,b) {
    document.write("Started Multiplication")
    return a * b * x
  3. Here also, mutating outer variable which is depending on an external variable.

    let x
    function mul(a,b) {
    x = 10
    return a * b * x

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.

let name = ["suprabha", "supi"]
function fullName(newName, name) {
let clonedName = [...name]
clonedName[clonedName.length] = newName
return clonedName
console.log(fullName("sumi", name)) // ['suprabha', 'supi', 'sumi']

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

Thanks for reading the article!

Hope you found this article useful. If you have any questions, feel free to reach out to me @suprabhasupi