JavaScript

####Function definition: 1. use var

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

2. function declaration

    function y() {
      console.log("y");
    }

---

####Closure

function sandwichMaker() {
  var magicIngredient = "peanut butter";
  function make(filling) {
    return magicIngredient + " and " + filling;
  }
  return make;
}

var f = sandwichMaker();
f("jelly");  // "peanut butter and jelly"
f("bananas");  // "peanut butter and bananas"

First Fact: Javascript allows you to refer to variables that were defined outside of the current function.

Second Fact: Functions can refer to variables defined in outer functions even after those outer functions have returned!

A more general-purpose sandwichMaker:

function sandwichMaker(magicIngredient) {
  function make(filling) {
    return magicIngredient + " and " + filling;
  }
  return make;
}

var hamAnd = sandwichMaker("ham");
hamAnd("cheese");  // "ham and cheese"
hamAnd("mustard");  // "ham and mustard"

var turkeyAnd = sandwichMaker("turkey");
turkeyAnd("Swiss");  // "turkey and Swiss"
turkeyAnd("Provolone");  // "turkey and Provolone"

A even more convenient way to construct closure, the function expression:

function sandwichMaker(magicIngredient) {
  return function(filling) {
    return magicIngredient + " and " + filling;
  };
}

Third Fact: Closures can update the values of outer variables because closures actually store references to their outer variables.

function box() {
  var val = undefined;
  return {
    set: function(newVal) { val = newVal; },
    get: function() { return val; },
    type: function() { return typeof val; }
  };
}

var b = box();
b.type();  // "undefined"
b.set(98.6);
b.get();   // 98.6
b.type();  // "number"