写得好的for循环在浏览器控制台中不起作用

So i was trying to make a password generator, and I want to get random letters and numbers from different arrays and assign to a variable which will now serve as the generated password, and now to increase the length of the characters in password i make to for loop add two random uppercase and lowercase letters and a number every time for 15 times and I'm pretty sure i wrote it well but when i tested in chrome's console it returned undefined immediately without the loop even running. Then i tried even the most basic loop in the console and the same thing happened, sometimes my browser just freezes like it's an infinite loop but it's not.So i was trying to make a password generator, a