var isLoaded = false

function exist(name) {
    $.ajax({
        type: 'HEAD',
        url: name,
        success: function() {
            isLoaded = true;
        },
        error: function() {
            exist(name)
        },
    });
}

exist();

function loadAfterExist() {
    // do some things
    if (isLoaded) {
        load();
    } else {
        setTimeout(loadAfterExist, 3000);
    }
}

loadAfterExist();