Here are different ways based on the type of element you are trying to get value using javascript:
For Input Elements (text, password, etc.)
HTML
<input type="text" id="myInput" />JavaScript
var inputValue = document.getElementById("myInput").value;For Select Elements
HTML
<select id="mySelect">
<option value="option1">Option 1</option>
<option value="option2">Option 2</option>
</select>JavaScript
var selectedValue = document.getElementById("mySelect").value;For Textarea Elements
HTML
<textarea id="myTextarea"></textarea>JavaScript
var textareaValue = document.getElementById("myTextarea").value;