摘錄：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>全局變量 局部變量</title>
</head>
<body>
<script language="javascript">
var number = -1; //文檔加載結束以后才執行
window.onload = function () { var height = 100; number = height; //重新賦值
} alert(number); //第一個執行，所以number沒變化 alert(-1);
function a() { alert(number); //這里number就已經在onload函數里被重新賦值了 alert(100);
} </script>
<input type="button" value="a" onclick="a()" />
</body>
</html>
自己測試
<!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>全局變量 局部變量</title>
</head>
<body onload="w()">
<button onclick="a()">anniu</button>
<script>
var num; alert(num); //第一個加載 alert(undefined)
function w() { num = 100; alert(num); //第三個加載 alert(100);
} alert("3" + num); //第二個加載 alert(3 undefined)
function a() { alert("4" + num); //點擊按鈕后加載 alert(4 undefined)
} </script>
</body>
</html>