window.addEventListener('keydown', function (event) {

    function modify(input) {
        var output = "Hello World";
        return output;
    }

    // check if enter was pressed
    if (event.keyCode === 13) {

        // prevent key press
        event.preventDefault();

        // Get the text field element
        var text_element = document.getElementById("searchInput");

        // Get text from the element
        var text_input = text_element.value;

        // Modify it
        var text_output = modify(text_input);

        // Writeback Code
        alert(text_output);

        // Set the text_element's value (input text into the field)
        text_element.value = text_output;
    }

}, false);