Validation form JavaScript

I am a beginner to JavaScript and I know about jQuery Validations however I do not want to use a foundation like jQuery just yet.

I am wondering if what I have done its good or if there is another way to do it other then jQuery?

I won’t post the html just the JS. It does work by the way! Just wondering if there is a better way!

function validateForm() {
  var userName = document.forms["register"]["username"].value;
  var passWord = document.forms["register"]["password"].value;
  var c_passWord = document.forms["register"]["c_password"].value;

  if (userName == null || userName == "") {
    alert("Your username can not be empty!");
    return false;
  } else if (userName.length < 3) {
    alert("Your username must be atleast 3 characters long!");
    return false;
  }

  if (passWord == null || passWord == "") {
    alert("Your password can not be empty");
    return false;
  } else if (passWord.length < 5) {
    alert("Your password must be more the 5 characters");
    return false;
  }
}

It mostly looks fine. Some things I would change:

var userName = document.forms["register"]["username"].value;
var passWord = document.forms["register"]["password"].value;

I would extract the repetition here to get the form. This makes the code cleaner and makes it simpler if you change the name of the form.

var form     = document.forms["register"];
var userName = form["username"].value;
var passWord = form["password"].value;

Here

if (userName == null || userName == "") {

You could just use

if (!userName == null) {

One last thing is that you probably want to check and remove leading and trailing spaces in the user name (and possibly in the password). There is a built in trim method you can use if you’re not targeting IE8.