JS BinUnderstand _.once function// source https://jsbin.com/mumoxip

understandOnce.js

// var chores = function () {
//   var tasks = ['laundry', 'dishes', 'vaccuum', 'dust'];
  
//   return {
//       doChores: function () {
//         console.log(tasks.pop());
//       },
//       addChores: function (task) {
//         tasks.push(task);
//         return task;
//       }
//   }
// }

// var house1 = chores();
// house1.doChores();
// house1.doChores();
// house1.doChores();
// house1.addChores('water plants');
// house1.doChores();

// var house2 = chores();
// house2.doChores();

var greeting = function () {
  return 'hello world!';
}

var greeting2 = function () {
  return 'konnichiwa';
}


var _ = {};

_.once = function (fn) {
  // keep track of when the function has been called
  var called = false;
  // keep track of the result
  var result;
  return function () {

  // and use the result if the function has already been called
    if (!called) {
      called = true;
      console.log('called!');
      result = fn();
    }
    return result;
  }
};

var foo = _.once(greeting);
console.log(foo());
console.log(foo());



// var foo2 = _.once(greeting2);
// foo2();