how to add a input value to a string in html code example

Example 1: javascript add value to input

document.getElementById("myText").value = "Johnny Bravo";

Example 2: insert value to html input with javascript variable

function product(a, b) {
   return a * b;
}
function setInputValue(input_id, val) {
    document.getElementById(input_id).setAttribute('value', val);
}