How to set the value of a select box element using JavaScript?

In JavaScript, selectedIndex property is used to set the value of a select box element. The selectedIndex property sets or returns the index of the selected value in a drop-down list. Syntax:

Set index: selectObject.selectedIndex = number Return index: selectObject.selectedIndex

Note: Integer number is used for Index at the value place. Example-1: Select a index number.

html

<!DOCTYPE html>
<html>
 
<body>
 
    <h1 style="color: green;">
            GeeksforGeeks
        </h1>
 
    <select id="mySelect">
        <option>football</option>
        <option>Basketball</option>
        <option>Hockey</option>
        <option>Swimming</option>
    </select>
 
    <p>
      Click the button to select the
      option element with index "2".
  </p>
 
    <button onclick="myFunction()">
      click me
  </button>
 
    <script>
        function myFunction() {
            document.getElementById(
              "mySelect").selectedIndex = "2";
        }
    </script>
 
</body>
 
</html>

Output: Before clicking on button, it stays on index 0: After clicking on button, it go on index 2: Example-2: If you select selectedIndex = “-1”; it will deselect all the elements of selectbox.

html

<!DOCTYPE html>
<html>
 
<body>
 
    <h1 style="color: green;">
            GeeksforGeeks
        </h1>
 
    <select id="mySelect">
        <option>football</option>
        <option>Basketball</option>
        <option>Hockey</option>
        <option>Swimming</option>
    </select>
 
    <p>
      Click the button to
      deselect options.
  </p>
 
    <button onclick="myFunction()">
      Click me
  </button>
 
    <script>
        //Here we delselect all the options
        function myFunction() {
            document.getElementById(
              "mySelect").selectedIndex = "-1";
        }
    </script>
 
</body>
 
</html>

Output: Before clicking on button, it stays on index 0: After clicking on button, all elements gets deselected: Example-3: If any element is not selected then this property returns -1.

html

<!DOCTYPE html>
<html>
 
<body>
 
    <h1 style="color: green;">
            GeeksforGeeks
        </h1>
 
    <select id="mySelect">
        <option>football</option>
        <option>Basketball</option>
        <option>Hockey</option>
        <option>Swimming</option>
    </select>
 
    <p>
      Click the button to
      deselect options.
  </p>
 
    <button onclick="myFunction()">
      Click me</button>
 
    <script>
        //Here we delselect all the options
        function myFunction() {
            document.getElementById(
              "mySelect").selectedIndex = "-1";
        }
 
        //here we are taking value of index
        function yourFunction() {
            var x = document.getElementById(
              "mySelect").selectedIndex;
           
            document.getElementById(
              "demo").innerHTML = x;
        }
    </script>
 
    <button type="button" onclick="yourFunction()">
        Display index
    </button>
 
    <p id="demo"></p>
 
</body>
 
</html>