Substring Validation

In this example form, a valid substring is sought. In its source code, observe:

Various comparison operators including ==, !=, >, >=, <, <=
The substring() method

Lab

Your task:

Make a copy of the example form and edit it as follows.
Test for valid credit card prefixes for MasterCard, Discover, and JCB.
Note: the syntax for string comparison is the same as for numbers. Quotation marks are optional.

Solution

Don't peek unless you're stuck!