Filter 2nd field drop-down list based on 1st field's value; JS pls.?

Hello,

I have 3 drop-down fields (3rd is a hidden field), they and their dorp-down list box entries are as follows,

1) Zone - US, EU

2) Area - New York, Houston, Missouri, Germany, France

3) Hidden_Area_Field - US$New York, US$Houston, US$Missouri, EU$Germany, EU$France

(2nd field's entries and 3rd field entries content & ORDER is same, except adding ZONE as prefix with a seperator of $)

Now, my requirement is, if user selected EU in 1st field, then the 2nd field's list box should fill with ONLY Germany, France

In the same way, if user selected the US in 1st field, then immediately the 2nd field's list box should filll with New York, Houston, Missouri

Just for example, I gave small # of entries, but actually i have big list

Pls. help me by providing the Java Script for my requirement pls.?

Thank you