JavaScript Fundamentals – The Pure Function

Once we start building our first algorithms, comes the desire and the need to write more maintainable code. This is where pure function comes into play.

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What is a so-called “pure” function?

This is a function that will not generate any side effects; that is to say, it will not modify anything outside its scope.
Examples:

• no modification of global variables,
• no modification of objects passed as arguments,
• no changes in the user interface,
• no network calls, etc.

It is more readable, predictable (reduces errors) and easy to test and debug. With the same parameters it returns the same result.

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Examples of pure functions

Let's take the following example to calculate an average:

const calculateAverage = (numbers) => {
  if (numbers.length === 0) return 0

  const totalSum = numbers.reduce((sum, num) => sum + num, 0)
  return totalSum / numbers.length
}


const scores = [80, 90, 75, 85, 95]
const average = calculateAverage(scores)

console.log(average) // 85

But without knowing it you are probably already using pure functions thanks to JavaScript methods like .toUppercase() which does not modify the original string, but returns a new uppercase string:

const text = "hello"

const upperText = text.toUpperCase()

console.log(upperText) // "HELLO"
console.log(text) // "hello" > la chaîne d'origine n'a pas été modifiée

And There you go, you know everything about pure functions :)

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