Prevent the default action of an event

1. Return `false` for the `on<event>`

ele.onclick = function(e) {
    // Do some thing
    ...

    return false;
};

It's same if you the inline attribute:

<form>
    <button type="submit" onclick="return false">Click</button>
</form>

I don't recommend this approach because

Returning false just doesn't make sense
It doesn't work with the addEventListener() method

2. Use the `preventDefault()` method

This method works with inline attribute

<button type="submit" onclick="event.preventDefault()">Click</button>

and event handlers:

ele.onclick = function(e) {
    e.preventDefault();

    // Do some thing
    ...
};

ele.addEventListener('click', function(e) {
    e.preventDefault();
    ...
});

Use cases

Don't follow a link when clicking it. We often use this when creating tabs.
Don't submit the form when clicking its submit button. For example, we want to validate the form first.
Don't open a file or download the file when dragging and dropping a file to a given area.
Show a custom context menu when right-clicking an element.

See also

Prevent body from scrolling when opening a modal

Prevent the page from scrolling to an element when it is focused