JavaScript is case sensitive

One very important point that we need to be clear about is that JavaScript is a case-sensitive language. You must be very careful about this because it can introduce some difficult to track down bugs in your code. All variables, keywords, functions, and identifiers must be typed with a consistent capitalization of letters. This gets even more confusing when you consider that HTML is not case sensitive.

This tends to be a stumbling block for new JavaScript developers. I have created three variables, all with the same spelling, but because they do not follow the same capitalization pattern you end up with three different variables as follows:

var myName = 'Eric';
var myname = 'John';
var MyName = 'Joe';