A pure function is a function that always returns the same result when given the same arguments, without side effects.
Example:
function double(x){
return x * 2;
}
// Will always return 4 when x is 2.
// Will always return 8 when x is 4, etc...function randomise(y){
return y * Math.random();
}
// Not a pure function because Math.random() always returns a different number.