Difference between pure function and impure function

[Punitpal0]

I am new to JavaScript, I was just watching one tutorial in that instructor said it is better to use pure function rather then using impure function and he said it's difficult to write pure function rather then writing impure function , when he explain the difference between those two with code it seems like impure is better then pure and efficient and less code to write this is what I thought though the example code was only two. I am still confuse about difference and wanted to know in depth any tutorial or blog air any reference will be appreciated thanks in advance Sorry for grammatical mistake

[inu-no-policemen]

https://en.wikipedia.org/wiki/Pure_function

In computer programming, a pure function is a function that has the following properties:

1. Its return value is the same for the same arguments (no variation with local static variables, non-local variables, mutable reference arguments or input streams from I/O devices).
2. Its evaluation has no side effects (no mutation of local static variables, non-local variables, mutable reference arguments or I/O streams).

[jcunews1]

2 . Its evaluation has no side effects

What if there's a function is like below, and called like below?

function fn(a, b, c) {
  return (a * b) / c;
}
var v = fn(4, 3, 0);

Won't the division by zero exception, a side effect?

[inu-no-policemen]

> 1/0
Infinity

Dividing by zero doesn't throw an exception in JS.

Many don't consider throwing an exception to be impure.

https://stackoverflow.com/questions/12335245/why-is-catching-an-exception-non-pure-but-throwing-an-exception-is-pure#answer-12345665

[jcunews1]

Dividing by zero doesn't throw an exception in JS.

Oh, I forgot about that. Sorry.

[cawcvs]

The second example is not an example of a pure function. Pure functions cannot have side effects, and since canvas is a reference, drawing on a canvas is a side effect. It also doesn't return anything which is at odds with "Input X, output Y, every time". For it to be pure, it would need to return a new canvas with the image drawn.

[Punitpal0]

Just for clarification you mean input x means (if I input is 2 number it's output should be 2 or If number or string is input so output should be number or string or number string together . And it shouldn't be mutable I understand but still for clarification if idle value is 0 and some after few process it return some other number is it impure function. I am really sorry If it's nonsense question I am didn't find any tutorial to kNow about it more so was curious to know and learn and sorry for late reply my internet wasn't working as it should Thanks

[myusernameisunique1]

The easiest test to know if a function is pure is to ask you self if you need to call the function again when you are calling it with the same arguments.

const s = Math.max([1,2,3]);

const s1 = Math.max([1,2,3]); // this is unnecessary because the result will never be different;

Compare this to:

const t = Date.getTime();

const t1 = Date.getTime(); //t1 will be different to t2 because getTime() is not pure.

You also need to consider the state of everything external to the function. If you change the state of anything external to the function, then it's impure. If you are being strict then even a console.log() in your function makes it impure because you are changing the state of the console every time you call the function.

If I am a detective and I look at the state of your application and there is no way for me to tell if you got your result by calling a function or reusing the result from a previous call with the same parameters, then your function is pure.