how to change the text value in javascript code example

Example 1: javascript set input field value

document.getElementById("myInputID").value = "My Value"; //set value on myInputID

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);
}