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John Peter
Pune, Maharashtra, India
How to validate combobox using Javascript and HTML with Example
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In this Example explained about Select Combobox validation using Javascript.

options attribute returns all the options of select component and selectedIndex returns the current option index. Index started with 0 likewise for 0,1,2,3 to <empty>,Siva,Raja and Roja respectively
HTML
Username:
<select name="slt_usernames" id="slt_usernames">
 <option></option>
 <option value="1001">Siva</option>
 <option value="1002">Raja</option>
 <option value="1003">Roja</option>
</select>
<button onclick="javascript:validate();">Validate</button>
JAVASCRIPT
function validate(){
    if (document.getElementById("slt_usernames").selectedIndex == 0){
       alert("Select the username");
    }
    else {
       alert(document.getElementById("slt_usernames").options[document.getElementById("slt_usernames").selectedIndex].value);
    }
}
OUTPUT 
DEMO 
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