Ajax不是功能

I'm getting this problem with my jQuery when I type something on my input type.

There is my js:

$(document).ready(function(){
    $("#cif").keyup(function(){
        var name = document.getElementById("cif").value;

        console.info(name);

        $.ajax({
            type: 'post',
            url: '<?php echo site_url(); ?>register/cif',
            dataType: 'json',
            data: {
                cif:name,
            },
            success: function (response) {
                $('#raosocial').html(response);
                if(response=="OK"){
                    return true;    
                }
                else{
                    return false;   
                }
            }
        });
    });
});

I tried without $.ajax but syntax error appeared

Make sure you aren't using the slim version of jQuery that doesn't include $.ajax().Download the full jQuery here