Discussion: question 77 need clarification

[RidgeA]

Question: https://github.com/lydiahallie/javascript-questions/blob/master/README.md#77-is-this-a-pure-function

I could imagine a situation where the function won't be pure.

let x = {
  [Symbol.toPrimitive]: () => {
    console.log('side effect');
    return 1
  }
}

let y = {
  [Symbol.toPrimitive]: () => {
    console.log('another side effect');
    return 2
  }
}

function sum(a, b) {
  return a + b
}

sum(x, y);

In this case, sum function will return 3, but all console.log are executed.
Obviously, this is a corner case, but....

[vinodkumartheking]

To best of my knowledge, a pure function is something which will return same result for same inputs, more kind of mathematical formula, !

[RidgeA]

That's true, but it is not enough.
Another mandatory property is the absence of side effects.
https://en.m.wikipedia.org/wiki/Pure_function

console.log is a side effect

Update:
Also, it is possible using the same trick with valueOf/Symbol.toPrimitive violate the first principle:

let x = {
  [Symbol.toPrimitive]: () => {
    console.log('side effect');
    return Math.random();
  }
}

[vinodkumartheking]

@RidgeA Yes, i agree. the above code now become not a pure function ! Thanks for the explaination.

[Lionad-Morotar]

hhh~ I just wanted to create an issue.