What

Fixes a bug while hydrating wp-class directives. It happens when the class attribute is not defined―something that occurs when no class is server-side rendered. E.g., the following HTML

<div wp-class:foo="state.falseValue"></div>
<button wp-on:click="actions.toggleFalseValue">Toggle</button>

currently throws this error:

Also, when the button is clicked, the application crashes.

Why

Directives should take care of those attributes they affect.

In this case, developers shouldn't be forced to write an empty class to make the wp-class directive work as expected.

How

Simply using an empty string when element.props.class is undefined.