HTMLInputElement: indeterminate property

The indeterminate property of the HTMLInputElement interface returns a boolean value that indicates whether the checkbox is in the indeterminate state. For example, a "select all/deselect all" checkbox may be in the indeterminate state when some but not all of its sub-controls are checked. The indeterminate state can only be set via JavaScript and is only relevant to checkbox controls.

It is unrelated to the HTMLInputElement.checked property, and an indeterminate checkbox can be either checked or unchecked. Being indeterminate only affects the checkbox's appearance (see example below), not its presence when submitted (which is controlled by the checkedness).

<input type="checkbox" id="indeterminate-checkbox" />
<label for="indeterminate-checkbox">Indeterminate checkbox</label>

const checkbox = document.getElementById("indeterminate-checkbox");
checkbox.indeterminate = true;