1、自调用匿名函数
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<script type="text/javascript">
(function(){
alert("好好学习，天天向上")
})();
</script>
</head>
<body>
</body>
</html>
返回值：好好学习，天天向上
2、 当事件发生时调用
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<script type="text/javascript">
window.onload=function(){
alert("好好学习，天天向上")
};
</script>
</head>
<body>
</body>
</html>
返回值：好好学习，天天向上
3、函数中调用
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<script type="text/javascript">
setTimeout(function(){
alert("好好学习，天天向上")
},3000) //延迟3000毫秒运行
</script>
</head>
<body>
</body>
</html>
返回值：好好学习，天天向上