prevent refresh of page when button inside form clicked

Add type="button" to the button.

<button name="data" type="button" onclick="getData()">Click</button>

The default value of type for a button is submit, which self posts the form in your case and makes it look like a refresh.

Let getData() return false. This will fix it.

<form method="POST">
    <button name="data" onclick="return getData()">Click</button>
</form>

All you need to do is put a type tag and make the type button.

<button id="btnId" type="button">Hide/Show</button>

That solves the issue

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