how to make function which related comboboxes option

Question

in my html page there are three form input has been related each other. first input as combobox with option:

• TCA
• Intasept second and third input as text type.

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

on simple logic.

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

this will continue with interval 120. ex:

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

if in first input = Intasept (selected option by user).

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

ex:

• second input : 02: 03 and so on.
• third input : 32-64: 65-96 and so on.

help me how to make this function in javascript. this, what i have tried but this not run

var interval, step;
$("#first").change(function (e) {
    if (this.option:selected) interval = { // map of input value attributes to 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) + "-" + ((interval * step + interval)-interval));
}

http://jsfiddle.net/n8pSt/

Answer 1

http://jsfiddle.net/QXFKL/5/

if (this.option:selected) interval = { // map of input value attributes to interval values
    "TCA": 120,
    "intasept": 32
}[this.value];

but the value that you are getting is "Intesept" Casesensitive

also your if didnt work so I've removed it