how to use the value of an input field in javascript code example

Example 1: Javascript get text input value

var inputValue = document.getElementById("myTextInputID").value;

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);
}