閉包中變量值的改變

1.對於閉包中的變量是怎麼回事？

function makeCounter() {
    var i = 0;
    console.log(i);
    return function(){
        console.log(++i);
        console.log(i);
    };   
}

//記住：`counter`和`counter2`都有他們自己的變量 `i`

var counter = makeCounter();
counter();//1
counter();//2

makeCounter();

var counter2 = makeCounter();
counter2();//1
counter2();//2

2.異步執行的順序是怎樣的？

for (var i = 0; i < 5; ++i) {
  setTimeout(function () {
    console.log(i + ' ');
  }, 100);
}

for (var j = 5; j < 10; ++j) {
  setTimeout((function (num) {
    console.log(num + ' ');
  })(j), 100);
}

for (var k = 10; k < 15; k++) {
	
	(function(num){
		setTimeout(function () {
    		console.log(num + ' ');
  		}, 100);

	})(k);	
}

for (var n = 15; n < 20; n++) {
	
	add(n);
}

function add(num){
	setTimeout(function(){
		console.log(num);
	});

輸出的結果是 5,6,7,8，9,15,16,17,18,19,5，5，5，5，5,10,11,12,13,14