javascript-pour-les-jedis-episode-1-snippet-15.js

function every(list, callback) {
  for (let i = 0; i < list.length; i += 1) {
    callback.call(list, i);
  };
}

every(['a', 42, false, {}], (index) => {
  console.log('1', this); //=> ['a', 42, false, {}]
  console.log('2', this[index]); //=> 'a', 42, false, {}
  console.log('3', this[index + 1]); //=> 42, false, {}, undefined
  console.log('4', this[index - 1]); //=> undefined, 'a', 42, false
});