function nonStrictSum(a, b) {

  // non-strict mode

  console.log(this === window); // => true

  return a + b;

}

function strictSum(a, b) {

  'use strict';

  // strict mode is enabled

  console.log(this === undefined); // => true

  return a + b;

}

// nonStrictSum() is invoked as a function in non-strict mode

// this in nonStrictSum() is the window object

console.log(nonStrictSum(5, 6)); // => 11

// strictSum() is invoked as a function in strict mode

// this in strictSum() is undefined

console.log(strictSum(8, 12)); // => 20