html - How to make function on related form in javascript -

in html page there 3 form input has been relataed each other. first input combobox option:

• tca
• intasept second , third input text type.

if first input fill "tca" when user input in second input "01" in third input automaticly filled "1-120". if first input fill "intasept" when user input in second input "01" in third input automaticly fille "1-32".

on simple logic.

• first input = "tca" (selected option user).
• second input = "01" (manually input user).
• third input = "1-120" (automaticly input).

this continue interval 120. ex:

• second input: 02: 03 , on.
• third input : 121-240: 241-360 , on.

if in first input = intasept (selected option user).

• second input = 01 (manually input user).
• third input = 1-32 (automaticly input).
• this continue interval 32.

ex:

• second input : 02: 03 , on.
• third input : 32-64: 65-96 , on. me how make function in javascript.

using jquery, simple script it:

jquery(function($) {     var interval, step;     $("input[name=firstcombobox]").change(function(e) {         if (this.checked)             interval = { // map of input value attributes interval values                 "tca": 120,                 "intasept": 32             }[this.value];         update();     });     $("#secondinput").on("change keyup input paste", function(e) {         step = parseint(this.value, 10);         update();     });     function update() {         if (isnan(interval) || isnan(step)) return;         $("#thirdinput").val((interval*step+1)+"-"+(interval*step+interval));     } }); 

demo @ jsfiddle.net

you might not use jquery else wait domready , attaching events, , might need adapt actual dom structure (input names/ids), should give idea how should done.