What does this.optional(element) do when adding a jQuery validation method?

Question

Please see the documentation:
https://jqueryvalidation.org/jQuery.validator.addMethod/

I wonder what this.optional(element) does. I created two forms to test:
Form1 and Form2 — one with this.optional(element) and the other without it. Theoretically speaking and according to a couple of comments on this answer by Andrew Whitaker:

all this.optional does is say "if the field is optional, return true if it is blank"

and

The this.optional check is basically checking to see if the field is blank or not before evaluating whether or not it meets the rule.

But in action I see no difference in how Form1 and Form2 work. Please help me understand the difference in action.

Answer 1

OK... so in your examples, the field is never blank in either form. Either it has a placeholder value, or an attempt at an email address. The whole point of this.optional(element) is to immediately return true if the element is blank AND it is not required.

So if you had these two methods:

jQuery.validator.addMethod("BOB", function (value, element) {
    return this.optional(element) || 
        element.value === 'BOB';
}, 'You did not enter BOB');

jQuery.validator.addMethod("mustbeBOB", function (value, element) {
    return element.value === 'BOB';
}, 'You did not enter BOB');

Adding a class of BOB required would be the same as entering a class of mustbeBOB . Compare that to having a class of BOB which would allow for a blank or "BOB", vs a class of mustbeBOB which will only pass validation with a value of BOB , blank would fail. Does that make more sense?

Answer 2

this.optional is intended to be used in general-purpose validation methods, which might be used with required or optional elements. It allows them to skip all their own checks if the field is not filled in. If the field is optional and blank, the method calling this.optional returns successfully immediately.

By using this, the method can assume that the value is non-empty, which can simplify the rest of its coding.