浏览器的事件处理机制

console.log(10);

for(var i = 0; i <5; i++) {

setTimeout(function () {

console.log(i);

},4000);

}

console.log(30);

10

30

5个3-----主线程结束的后4秒后打印

window.onload = function () {

console.log(10);

document.getElementById(‘btn‘).onclick = function () {

console.log(20);

};

console.log(30);

};

10

30

20----点击的时候打印

var xhr = new XMLHttpRequest();

xhr.open();

xhr.send();

console.log(1);

var data = null;

xhr.onreadystatechange = function(){

console.log(2);

data = xhr.responseText;

}

console.log(data);

console.log(3);

1

3

2