Pure function
A pure function is a deterministic function. This means when a same input is passed every time, the function will return same output. In mathematical terms it is nothing but a well defined function.
A pure function will have the following properties
It depends only on its own arguments.
It wont try to change variables out of its scope.
It doesn't produce any side effects.
The following example is not a pure function because rather than depending on its own variables, the function is depending on variables(val1 and val2) that are out of its scope.
Example
<html>
<body>
<script>
let val1 = 6;
let val2 = 4;
function pure() {
return val1 * val2;
}
document.write(pure());
</script>
</body>
</html>output
24
In the following example the function, even though depending on its own arguments, is also depending on some outside variables(val). So it is not a pure function.
Example
<html>
<body>
<script>
function pure(arg) {
let val = 100;
return val* arg;
}
document.write(pure(2));
</script>
</body>
</html>output
200
The following example displays a pure function because there are no outside variables involved while the function is performing its tasks. The function returns the same output when a same parameter is passed to it
Example
<html>
<body>
<script>
function pure(arg) {
return 4 * arg;
}
document.write(pure(2));
</script>
</body>
</html>output
8