Am I 'allowed' to modify props in the constructor?

Question

They say you shouldn't modify the props in a React Component. Does that extend to modifying them in the constructor?

Specifically,

export default class BookingForm extends React.Component {

    constructor(props) {
        // am I allowed to modify `props` here?
        super(props);
    }
}

To be perfectly clear, I'm aware that JavaScript will allow me to do so, I'm asking if this is a bad design pattern that will cause me headache in the future.

I want to re-format some of the props upon initialization; they won't change again after that.

Answer 1

To re-iterate what zerkms pointed out to me, the answer is no, you are not allowed to modify props, even in the constructor.

super() expects the exact same props that you were given, and even if you try to cheat the system by supplying something different, they will be overwritten immediately after the constructor. Ergo, you cannot modify this.props.

Even if you try to modify the props object to add an extra property, you will see an error like this:

TypeError: Can't add property bacon, object is not extensible

So you are literally unable the modify the props in the constructor, even if you wanted to.

However, you can set new [JavaScript] properties, e.g.

this.myReformattedProperty = format(props.oldProperty)