Question

we have an input type="text" user prints into it some string. each time he inputs empty space char we want to call some function. How to do such thing with jQuery?

Answer 1

$(input).keydown(function (e) {
    if ( e.which === 32 ) {
        // do your thing
    }
});

where input is a reference to your INPUT element.

Live demo: http://jsfiddle.net/uHpLg/