What is an example of an impure function in JavaScript

Question

Having seen a lot of pure functions and how they have no side effects, what would be an example of an impure function, which is always been antagonized as unstable and major source of error?

Answer 1

For example an impure function that has a side effect on a variable outside of its own scope:

var count = 0;

function increaseCount(val) {
    count += val;
}

Or a function that returns different values for the same input because it evaluates a variable that is not given as parameter:

var count = 0;

function getSomething() {
    return count > 0;
}

Answer 2

A pure function doesn’t depend on and doesn’t modify the states of variables out of its scope.

Concretely, that means a pure function always returns the same result given same parameters. Its execution doesn’t depend on the state of the system.

var values = { a: 1 };

function impureFunction ( items ) {
  var b = 1;

  items.a = items.a * b + 2;

  return items.a;
}

var c = impureFunction( values );
// Now `values.a` is 3, the impure function modifies it.

Here we modify the attributes of the given object. Hence we modify the object which lies outside of the scope of our function: the function is impure.

var values = { a: 1 };

function pureFunction ( a ) {
  var b = 1;

  a = a * b + 2;

  return a;
}

var c = pureFunction( values.a );

we simply modify the parameter which is in the scope of the function, nothing is modified outside!

var values = { a: 1 };
var b = 1;

function impureFunction ( a ) {
  a = a * b + 2;

  return a;
}

var c = impureFunction( values.a );
// Actually, the value of `c` will depend on the value of `b`.
// In a bigger codebase, you may forget about that, which may 
// surprise you because the result can vary implicitly.

Here, b is not in the scope of the function. The result will depend on the context: surprises expected!

var values = { a: 1 };
var b = 1;

function pureFunction ( a, c ) {
  a = a * c + 2;

  return a;
}

var c = pureFunction( values.a, b );
// Here it's made clear that the value of `c` will depend on
// the value of `b`.

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