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Pure vs Impure Functions

Sandra Spanik on March 18, 2021

Software engineering is full of jargon. Occasionally, to grasp the true meaning of the seemingly simplest of words, one must waddle through many mu...

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EECOLOR • Mar 24 '21

There are a few use cases where an implementation of a function could be considered 'not pure' while, from the outside, it is pure.

let complexValue = null
export function getComplexValue() {
  return complexValue || (complexValue = calculateComplexValue())
}

function calculateComplexValue() {
  // this is always the same
  ...
  return ...
}

export function mapValues(o, f) {
   return Object.entries(o).reduce(
    (result, [k, v]) => (result[k] = f(v, k, o), result),
    {}
  )
}

Toan Le Viet • Mar 19 '21

Yah I agree that Impure Function is also super useful in certain contexts. Sometimes, we use in-place solution to reduce the space complexity instead of creating copies with Pure Function

Robin Bauhn • Mar 19 '21

Nice article!
I think that it is worth pointing out could be that spread syntax only is pure for primary types. For objects and that sort of stuff it is only the reference that's being copied so any mutations would also mutate the orignal object.

Brad Beggs • Mar 18 '21

.sort() is also impure, sorting the input in-place.

Sandra Spanik • Mar 18 '21

Thanks for the suggestion, I've updated the list!

J. G. Sebring • Mar 19 '21

I'd like a way to flag pure functions like an annotation or similar. Last time I looked this up I didn't find any so I'm throwing this out - anyone doing this, and how?

example, jsdoc style

/**
* @pure
*/
add (x, y) {
 return x + y
}

Mathias Gheno Azzolini • May 25 '22

I would love that too.

Nikhil Singh • Apr 24 '21

Great article! I would like to add that writing a pure function is more about how much control/ confidence you have on that piece of code, rather than following a bunch of rules and assuming it as a pure function. By confidence/ control, I mean how sure are you that if a certain input is any given later in time, that piece of code will give the same output.

That's how I see pure functions.

Brad Beggs • Mar 22 '21

And in particular, you would need to assign the result of using the rest operator to a new variable get the sorted array?

For example:
const array = [1,2,6,5,4]
let newArray = [...array].sort((a,b) => a-b )

console.log(newArray) // [1, 2, 4, 5, 6]
console.log(array) // [1,2,6,5,4]

Sandra Spanik • Mar 20 '21 • Edited

Great, I want to be exact in the language I use so thank you for commenting :) I edited “considered pure” to “typically associated with pure functions”.

As for the definition, I found it elsewhere. Before I update it, let me try and see if I’m parsing what you’re saying correctly: you’re saying that while my definition describes some side-effects, it doesn’t describe ALL side-effects, because there are some which are indeed related to computing the final output. Is that right?