yangsu

console.log(y === undefined); // "true"
var y = 3;
 
// will return a value of undefined
var myvar = "my value";
(function() {
  console.log(myvar); // undefined
  var myvar = "local value";
})();

yangsu

// x is defined in the body of the if, but can be referenced outside
if (true) {
  var x = 5;
}
console.log(x);

(function () {
  var x = 10;
  console.log(x); // 10
})();

yangsu

foo(1, 2, 3); // Works fine because we declared function using
              //  the function literal at the bottom
bar(1, 2, 3); // => ReferenceError: bar is not defined

// The functions name is optional
// Here we are storing an anonymous function in a variable bar
var bar = function (a, b, c) {
  // ...
};

yangsu

foo(4, 5);
function foo (a, b) {
  var x = a + b;  // a = 4, b = 5, x = 9
  function bar (c) {  // c = 9
    return a + b + c + x; // returns 18
  }
  return (14 * bar(a + b) / x) || 42;
}

yangsu

// this refers to the containing object's context
var obj = {
  value: 0,
  increment: function (inc) {
    inc = inc || 1;
    this.value += inc;
  }
};
obj.increment();    // obj.value = 1
obj.increment(2);   // obj.value = 3

yangsu

// this refers to the global object (a mistake in the language design)
obj.quadruple = function () {
  var that = this;  // common idiomatic workaround for preserving context
  var helper1 = function () {
    that.value = that.value * 2;
  };
  var helper2 = function () {
    this.value = this.value * 2;
  };
  helper1();

yangsu

// this refers to the newly created object returned by new
var Counter = function (init) {
  this.value = init || 0;
}
Counter.prototype.increment = function (inc) {
  inc = inc || 1;
  this.value += inc;
}
var anotherObj = new Counter();
var bad = Counter();  // Don't do this, value gets attached to the global object

yangsu

var incrementer = function (inc) {
  inc = inc || 1;
  this.value += inc;
};

var foo = { value: 5 };
incrementer.apply(foo, [1]);  // foo.value = 6
incrementer.call(foo, 2);     // foo.value = 8

yangsu

function sumParams () {
  var actualArrayOfArgs = Array.prototype.slice.call(arguments),
      sum = 0;
  actualArrayOfArgs.forEach(function (arg) {
    sum += arg;
  });
  return sum;
}
sumParams(1, 2, 3); // 6

yangsu

var add = function (a, b) {
  if (typeof a !== 'number' || typeof b !== 'number') {
    throw {
      name: 'TypeError',
      message: 'add needs numbers'
    }
  }
  return a + b;
}
try {