1、变量未定义：
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>Document</title> </head> <body> <script> console.log(a); // undefined </script> </body> </html>
2、变量定义了但未赋值：
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>Document</title> </head> <body> <script> var a; console.log(a); // undefined </script> </body> </html>
3、函数未传参：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<script>
function show(a){};
console.log(show()); // undefined
</script>
</body>
</html>
4、只有 return 没有值：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<script>
function show(){
return;
};
console.log(show()); // undefined
</script>
</body>
</html>
5、函数没有返回值：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<script>
function show(){
};
console.log(show()); // undefined
</script>
</body>
</html>