The only explanation to length being undefined is if inputData is not a string. You neglected to mention what type of input you're working with, but in any case, casting to string should solve the issue:

function checkInputData() {
    $('.mainSearchSubmit').live('click', function () {
        var inputData = $(this).parent().children().eq(0).val();
        inputData = String(inputData); // Cast to string
        console.log(inputData.length);
        //this returns undefined
        console.log(inputData);
        //and this returns text from inpu so i know there is data
    });
 }