impureFunction.js

var foo = 10;
var bar = 20;

function impureFunc(arg){
  arg = arg + bar * 2;
  return arg;
}

var result = impureFunc(foo);

pureFunction.js

var foo = 10;
var bar = 20;

function pureFunc(arg, arg2){
  arg = arg + arg2 * 2;
  return arg;
}

var result = pureFunc(foo, bar);