一、放在文件里
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>Title</title> <script src="commons.js"></script> </head> <body> </body> </html>
alert(123);
二、写在head里
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>Title</title> <script type="text/javascript"> alert(123); </script> </head> <body> </body> </html>
三、写在body里（一般不这么做）
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>Title</title> </head> <body> <script type="text/javascript"> alert(123); </script> </body> </html>
原文：https://www.cnblogs.com/staff/p/10100203.html
