Javascript functions and objects

examples.js

// follow along in the interpreter: node or browser developer console

// variable whose value is anonymous function
var square = function(num) {
  return num * num;
}

// named function
function cube(num) {
  return square(num) * num
}

// local vs global scope
// from eloquent javascript
var x = "outside";

var f1 = function() {
  var x = "inside";
};
f1();
console.log(x);

var f2 = function() {
  x = "inside";
};
f2();
console.log(x);

// functions within functions
// from eloquent javascript
function noisy(f) {
  return function(arg) {
    console.log("arg: ", arg);
    var val = f(arg);
    console.log("val: ", val);
    return val;
  };
}

// better noisy with apply
function betterNoisy(f) {
  return function() {
    console.log("args: ", arguments);
    var val = f.apply(null, arguments);
    console.log("val: ", val);
    return val;
  };
}

// "optional" arguments
// from eloquent javascript
function power(base, exponent) {
  if (exponent == undefined)
    exponent = 2;
  var result = 1;
  for (var count = 0; count < exponent; count++)
    result *= base;
  return result;
}

// simple closure
// from eloquent javascript
function myClosure(n) {
  var local = n;
  return function() { return local; };
}

// password authentication using a closure
// this is a closure because password is a local variable in the passwordChecker function
// yet that variable is accessible in the anonymous inner function

// returns a function which checks if the guess is equal to password
function passwordChecker(password) {
  return function(guess) {
    if (guess == password) {
      console.log('successfully authenticated');
    } else {
      console.log('failure.');
    }
  }
}

var authenticator = passwordChecker('cool');
authenticator('wrong'); // failure.
authenticator('cool'); // successfully authenticated

//----------------------------objects in JS-------------------------

// constructor to use with new
// when this function is called, an object is created
// with a prototype that can be accessed with Teacher.prototype
var Teacher = function(name, lecture) {
  this.name = name;
  this.lecture = lecture;
}

// don't forget the new keyword
var sherif = new Teacher('sherif', function() {
  console.log('what do you want to cover today? how about ajax?');
});

// because the new keyword was used, julian is associated with the Teacher prototype
// therefore, julian can use any methods and properties associated with Teacher.prototype

Teacher.prototype.grade = function() {
  console.log('you get an A');
}

// one way to write a constructor
// you don't get OOJS prototype functionality
var Mentor = function(name, lecture) {
  mentor = {};
  mentor.name = name;
  mentor.lecture = lecture;

  return mentor;
}

var amadou = Mentor('amadou', function() { console.log('enthusiastic encouragement'); });

Mentor.prototype.help = function() { console.log('read the error'); } // should apply to all mentors??

amadou.lecture() // works
amadou.help() // error

// new won't solve our problems here with the help function
var christianne = new Mentor('christianne', function() { console.log('jQuery is great'); })
christianne // { name: 'christianne', lecture: [Function] }

christianne.lecture() // works
christianne.help() // still doesn't work TypeError: christianne.help is not a function

// phase 3 content, read ahead if you are interested

// module using an immediately invoking function expression
// same password checking functionality as above
var soundcloud; // this is the module
(function(exports) {
  var API_KEY = 'secretkey';

  exports.authenticate = function(key) {
    if (key === API_KEY) {
      console.log("authentication successful");
    }
    else {
      console.log("go away spammer");
    }
  }
})(soundcloud = {});

soundcloud.authenticate('fake'); // go away spammer
soundcloud.authenticate('secretkey'); // authentication successful
soundcloud.API_KEY = 'evilkey'; // doesn't alter the API_KEY within the module. it's a new property that doesn't matter
soundcloud.authenticate('evilkey'); // does not work

// augmentation i.e. adding new properties to an existing "module"

(function(exports) {
    exports.playMusic = function() {
      console.log("boomshakalaka");
    };
    return exports;
}(soundcloud || {}));

soundcloud.playMusic();

// sub-modules i.e. module within a module
soundcloud.rapModule = (function(exports) {
  exports = {};
  exports.catchPhrase = 'yo yo yo';

  return exports;
}());

soundcloud.rapModule.catchPhrase;