Boolean props aim to behave like boolean attributes in HTML.

So, for example, the checked attribute of a native checkbox can be set in various ways:

<input type="checkbox" checked>
<input type="checkbox" checked="">
<input type="checkbox" checked="checked">

Vue handles boolean component props the same way. Imagine we have the prop checked: Boolean, then:

<MyCheckbox checked />
<MyCheckbox checked="" />
<MyCheckbox checked="checked" />

All of these will lead to the checked prop being set to true. The first two will both lead to checked being an empty string.

If you have a prop that can be either a string or a boolean then the order matters. For example, checked: [String, Boolean] would not convert string values to true, whereas checked: [Boolean, String] would.