JavaScript special

JavaScript.js

/*
 * http://dmitrysoshnikov.com/ecmascript/chapter-2-variable-object/ 
 */
alert(x); // function

var x = 10;
alert(x); // 10

x = 20;

function x() {}

alert(x); // 20

/* 
 * http://dmitrysoshnikov.com/ecmascript/es5-chapter-3-2-lexical-environments-ecmascript-implementation/ 
 * This is why, closures formed as function declarations (FD) save the VariableEnvironment component as their [[Scope]] property, 
 * and function expressions (FE) save exactly LexicalEnvironment component in this case. This is the main (and actually the only) 
 * reason of separation of these two, at first glance the same, components.
 */

var a = 10;
 
with ({a: 20}) {
 
  // FD
  function foo() { // do not test in Firefox!
    console.log(a);
  }
 
  // FE
  var bar = function () {
    console.log(a);
  };
 
  foo(); // 10!, from VariableEnvrionment
  bar(); // 20,  from LexicalEnvrionment
 
}
 
foo(); // 10
bar(); // still 20

/* http://www.2ality.com/2012/01/object-plus-object.html */

[] + [] // ''
[] + {} // [object Object] ('' + String({}))
{} + {} // NaN (empty block Number({}) )
({} + {}) // [object Object][object Object] (is an expression)
{} + [] // 0 (empty block + [] -> Number(''))

/* FDs */

/* FD must not be defined within a block, but each browser is this supporting in a different implementation */
if (true) {
  function foo() {
    console.log("if foo");
  } 
} else { 
  function foo() {
    console.log("else foo");
  } 
}

foo();

/* closure */
var x = "global";

var o = (function () { 
  var x = "local"; 
  function closure() {
    return x;
  } 
  var g = new Function('return x;'); // has always the global scope.
  return { 
    closure: closure, 
    g: g
  }; 
}());

o.closure();
"local"
o.g();
"global"

/* IE anomalies */

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

foo(); // function
bar(); // < IE 9: "function"; >= IE 9: "bar" is undefined;


// with the identical names
console.log(foo); // < IE 9: prints the definition of the function. >= IE9: undefined
var foo = function foo() {
  console.log("function");
}
foo(); // "function"