You can use JavaScript to validate an email address by using regular expressions. A regular expression is a sequence of characters that defines a search pattern. In this case, you can use a regular expression to check whether the email address entered by the user matches the pattern of a valid email address. Here’s an example of how to do it:

<form>
  Email: <input type="text" id="email">
  <input type="button" value="Submit" onclick="validate()">
</form>
<script>
  function validate() {
    var email = document.getElementById("email").value;
    var re = /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
    if (re.test(email)) {
      alert("Valid email address.");
    } else {
      alert("Invalid email address.");
    }
  }
</script>

In this example, when the user clicks the “Submit” button, the validate() function is called. This function gets the value of the “email” field, and check if it matches a regular expression pattern of a valid email address using the test() function. If the value matches the pattern, an alert is displayed to the user indicating that the email address is valid. Otherwise, it will show “Invalid email address.”

It is also a good practice to validate the email address on the server side as well, to ensure that the validation is done even if the user has disabled JavaScript.

Please note that this regular expression pattern is a basic one, and it might not cover all the possible variations of email addresses.

Also, you may use other libraries like email-validator or express-validator to perform email validation in JavaScript.

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