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Pure functions are functions that have two characteristics:
When a function changes the external state, it is said to produce side effects. Pure functions should not produce side effects.
When you pass a set of input values into a pure function, it should always return the same output value. This is easier to explain with an example.
Let’s say you have a function called sum that sums two numbers together.
const sum = (n1, n2) => n1 + n2
If you pass 3 and 5 into sum, you will get 8. It doesn’t matter how many times you run sum(3, 5). You will always get 8. The output remains unchanged given the same input.
sum(3, 5) // 8
sum(3, 5) // 8
sum(3, 5) // 8
sum(3, 5) // 8
An impure function’s value can change given the same input. Let’s say for example you have the following function that uses Math.random.
const random = _ => Math.random()
Math.random is a built-in JavaScript method that gives you a random value between 0 to 1. It changes each time you run it.
random() // 0.4656
random() // 0.0037
random() // 0.4346
random() // 0.4989
Pure functions cannot depend on variables in the external state. This is because the output value will be different if the external state changes.
let num = 5
const addValue = valueToAdd => num + valueToAdd
addValue(10) // 15
num = 100
addValue(10) // 110
You can still write functions that relies on external variables if you know they won’t change.
const num = 5
const addValue = valueToAdd => num + valueToAdd
addValue(10) // 15
addValue(30) // 35
addValue(10) // 15
Practice this with discretion.