https://twitter.com/andrestaltz/status/768825820647399424

rttw.js

// This function can be impure in JS context...
function sum(arr) {
  var z = 0;
  for (var i=0;i<arr.length;i++) {
    z += arr[i];
  }
  return z;
}

// sec 1: valueOf() impureness
let internalVal = 100;
function getRttwVal() {  
  return {
    valueOf: function() { return internalVal += 100; }
  };
}
const foo = [getRttwVal(), getRttwVal(), getRttwVal()]
console.debug(sum(foo)); // 900 
console.debug(sum(foo)); // 1800
console.debug(sum(foo)); // 2700

// sec 2: getter impureness (wara)
let internalVal2 = 0;
function getRttwVal2() {
    return {
        get '0'() {
            return internalVal2 += 1;
        },
        get '1'() {
            return internalVal2 += 3;
        },
        get '2'() {
            return internalVal2 += 7;
        },
        length: 3
    };
}

const foo2 = getRttwVal2();
console.debug(sum(foo2)) // 16
console.debug(sum(foo2)) // 49
console.debug(sum(foo2)) // 82

// Also note ES2015 Proxy can be used to do similar effect