jQuery.map(....).concat is not a function

Question

Why would the following jquery code sometimes throw the error "concat is not a function":

var myArray = $('div.foo')
    .filter(function() { return $(this).is('.something'); })
    .map(function() { 
        return [['a', 'b', $(this).val()]];
    });

return myArray.concat(anotherArray);

Answer 1

$().map() returns a jQuery object, not an array.
jQuery objects do not have a concat() method.

You need to call .get() to get a real array.