Closures, Callbacks, Scope

closures_callbacks_scope.js

Closure

a closure is the local variables for a function - kept alive after the function has returned

a closure is a stack-frame which is not deallocated when the function returns



// scope
function outer_scope() {
  var foo = 1;
  console.log ('foo', foo);

  function inner_scope() {
    var foo = 2;
    console.log ('foo', foo);
  }

  inner_scope();
}

outer_scope();

// callbacks
function makeNumbers(i, cb) {
  var num1, num2, num3;
  num1 = i;
  num2 = i*2;
  num3 = i*i;

  cb(num1, num2, num3);
}

makeNumbers(42, 
  function printAndAdd(a, b, c) {
    console.log(a, b, c);
    console.log('sum', a+b+c);
  } );


// closure 
var getName, getCharacterClass;

function healerPlz() {
  var name = 'Codex';
  var characterClass = 'Priest';

  getName = function() {return name;};
  getCharacterClass = function() {return characterClass};
}

healerPlz();
console.log(getName() );
console.log(getCharacterClass() );
console.log(name);

// callback
function retrieveDatabaseEntry(recordNum, cb) {
  var db = ['foo', 'bar', 'bletch', 'quux', 'bulbasaur', 'charmander', 'squirtle', '42'];
  var result = db[recordNum];

  cb(result);
}

function printEntry(val) {
  console.log('retrieved value: ', val);
}

retrieveDatabaseEntry(5, printEntry);

// scope 
var item = 'THE HOLY GRAIL'  // CAN'T TOUCH THIS
function building() {
  var item = 'whiteboards';
  console.log(item);

  room();
  function room() {
    var item = 'cookies';
    console.log(item);
  }
}

// closure
function getStats(a, b) {
  var vals = {
    min: (a < b) ? a : b,
    max: (a > b) ? a : b
  }

  return function() { return vals; };
}

console.log( getStats(42, 9000) () );
console.log( getStats(9000, 42) () );

// callback
function prepareAndDeliverFood(food, cb) {
  console.log('preparing', food);

  cb(food);
}

function deliverFood(food) {
  console.log(food, 'delivered.  Have a nice day!');
}

prepareAndDeliverFood( 'milk' , deliverFood );

// closure
function returnGreeting() {
    var text = 'Hello, World';

    var greetingFunction = function() {
      return text;
    }

    return greetingFunction;
}

var newGreetingFunction = returnGreeting();

console.log( returnGreeting()  );
console.log( newGreetingFunction()  ); // returns the value of "text"