This one stumped me for a bit when i was trying to use jQuery to populate checkboxes with a list of items pulled from a database using PHP.

The fact of the matter is that PHP’s in_array() returns a boolean whereas jQuery’s inArray() returns the index of the matching element, or if the element is not found it will return -1 or undefined, depending on the browser.

Problem

If you are used to using PHP like me jQuery’s inArray() can give you a headache when you are using the returned value of the function in an if-else condition. This is because -1 evaluates as true, when you really want it to evaluate as false.

Solution

Make it so that your if-else condition checks for a value >=0 (greater than or equal to zero). Since array’s don’t have negative indexes you’re safe doing it this way.

What did i learn from this? When working with a new function check the documentation for what type of data it will return.

jQuery.inArray Documentation

Example