Ability to call setState in the constructor

A design question raised in another bug: https://github.com/facebook/react/issues/5313#issuecomment-152668532

What is the ideal behavior? Why don’t we support setState() in the constructor? It seems better than having people manually add this.state = {...} in the constructor, since the current method encourages the imperative pattern that can’t be used elsewhere in the component. cc @sebmarkbage for an answer.

[sebmarkbage]

Even componentWillMount is an anti-pattern. Another scenario is callbacks to a parent in componentWillMount. Where it may reach into a ref before the ref is uninitialized. It causes all kinds of strange artifacts.

Better to just move it to didMount.

Side-effects that change state due to a component being rendered is even more of an anti-pattern and may break scheduling etc.

[jimfb]

Just talked with Sebastian, and we agreed that this is probably a pattern that we will never want to support. In most cases, calling setState in the constructor is an anti-pattern. The intended “story” for the future is to use property initializers once they land in ES7-ish, and then the this.state = {...} will go away.

I’ll leave this issue open for another day or two in case people want to continue the discussion.