摘录：https://bbs.csdn.net/topics/391110498?page=1
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>全局变量 局部变量</title>
</head>
<body>
<script language="javascript">
var number = -1;
//文档加载结束以后才执行
window.onload = function ()
{
var height = 100;
number = height; //重新赋值
}
alert(number); //第一个执行，所以number没变化 alert(-1);
function a() {
alert(number); //这里number就已经在onload函数里被重新赋值了 alert(100);
}
</script>
<input type="button" value="a" onclick="a()" />
</body>
</html>
自己测试
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>全局变量 局部变量</title>
</head>
<body onload="w()">
<button onclick="a()">anniu</button>
<script>
var num;
alert(num); //第一个加载 alert(undefined)
function w() {
num = 100;
alert(num); //第三个加载 alert(100);
}
alert("3" + num); //第二个加载 alert(3 undefined)
function a() {
alert("4" + num); //点击按钮后加载 alert(4 undefined)
}
</script>
</body>
</html>