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Discussion on: Functional Programming Principles in JavaScript

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jonrandy profile image
Jon Randy 🎖️ • • Edited

JS is fun...

const squareIt = (number) => {
    return number * number;
}
const myInput = { valueOf() { return performance.now() }}

console.log(squareIt(myInput)) // 6046681
console.log(squareIt(myInput)) // 6056521
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Pure(?) function - same input, different outputs 👍

Kind of a 'trick' - I know - but the input is identical each time. Because of this, in the strictest sense, it can be considered that functions such as these are not pure - as there are possible inputs that will wreck the purity.

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architectak profile image
Ankit Kumar • 

const squareIt = (number) => {
    return number * number;
}
const myInput = { valueOf() { return performance.now() }}

console.log(performance.now()) // 132161.5
console.log(squareIt(myInput)) // 17466767811.611576
console.log(performance.now()) // 132162
console.log(squareIt(myInput)) // 17466847108.83685
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JS is fun, since input is changing its value on each call, hence output is different

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jonrandy profile image
Jon Randy 🎖️ • • Edited

But the input to the function is the same. The calculation differs because of coercion that happens as a feature if JS. The inputs themselves are unchanged. The input in this case is an object with a single method.

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architectak profile image
Ankit Kumar •

In input of function is function that keeps changes its value, then that function can not be considered as pure function, right ?

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jonrandy profile image
Jon Randy 🎖️ • • Edited

The input is an object, but it's value upon coercion to a number differs on each coercion. In a very strict sense, such a simple function could be considered impure. To make it strictly pure in JS, you could add a bunch of code checking types etc.

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jackmellis profile image
Jack •

Put simply, the input itself is impure. The valueOf getter uses performance.now which is external state (calling valueOf with the same inputs does not return the same result every time). When you introduce impure inputs then you’ve already broken the fp paradigm.