Rule suggestion: no-state-in-constructor

[hornta]

INCORRECT

class MyComponent extends Component {
  constructor(props) {
    super(props);

    this.state = {
      counter: props.initialValue
    };
  }
}

CORRECT

class MyComponent extends Component {
  state = {
    counter: this.props.initialValue
  };
}

Why?
It may reduce boilerplate by not having to define a constructor just for initializing the state.

What do you think about this?

[ljharb]

Since class fields are still stage 3, and not yet supported by default eslint, I'm not sure this is worth having as a rule yet - but this sounds like a great rule.

[fatfisz]

Just today my workmate suggested a similar idea - forbidding constructor in React components altogether.

The rationale for this is that it's easy to forget to call super with props, and according to React docs this may result in a bug:

When implementing the constructor for a React.Component subclass, you should call super(props) before any other statement. Otherwise, this.props will be undefined in the constructor, which can lead to bugs.

Then we were wondering what are the usecases for constructors, and there it is:

• Initializing properties on this from the arguments (which includes state): since React only passes one argument to the constructor, props, which is also available as this.props when initializing through class fields, we have this covered.
• Side effects: we don't want any in the constructor, so we ignore that.

So even with constructor we have all bases covered. As for the class fields, aren't they quite popular already in the React community? This rule wouldn't need to be turned on by default, so the decision would be up to devs. If they wanted to turn this rule on, they'd need to support class fields anyway - because otherwise where would they initialize state without a constructor?

[krawaller]

(@fatfisz) The rationale for this is that it's easy to forget to call super with props

Note that there is a rule to catch omitted super calls.

[fatfisz]

It doesn't enforce that you call super with the proper arguments though (not just super(), but super(props)), and that's my problem with using the constructor in React components.

[krawaller]

That's true. Issue #626 in this repo suggests such a rule, but it hasn't happened yet.

[lukyth]

My employer happens to need this kind of rule as well. Is it okay for me to try working on this?

My idea is that the rule name is state-in-constructor. It could receive an option, always or never (default is always). If it's always, state initialization should be in a constructor. If it's never, it should be in a class property.

The reason I omitted no in the rule name is because no-state-in-constructor with an option never is a double negation and it seems confusing to me.

[ljharb]

@lukyth that sounds reasonable to me! want to make the PR?

[lukyth]

Yeah. Let me try this out. I'll open a PR once I got something.