Check out example codes for "onchange on select tag". It will help you in understanding the concepts better.

Code Example 1

<select id="comboA" onchange="getComboA(this)">
  <option value="">Select combo</option>
  <option value="Value1">Text1</option>
  <option value="Value2">Text2</option>
  <option value="Value3">Text3</option>
</select>

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