Replace UNSAFE_componentWillMount with constructor

[voodoocreation]

This could be remedied by replacing UNSAFE_componentWillMount with a constructor definition, as that is the modern equivalent of using the now-deprecated componentWillMount. It will also avoid limiting your supported versions of react, as it's both backwards and forwards compatible.

react-side-effect/src/index.js

Line 70 in f1e0207

UNSAFE_componentWillMount() {

[sabarasaba]

any chance we can get this merged in @gaearon ? this package is heavily used by react-helmet and im sure im not the only one bugged by all the warnings it generates 🐱

[andreyluiz]

@sabarasaba, there is https://github.com/iest/react-body-classname which is potentially impacted by this too.

[sophiebits]

Renaming to constructor isn't the correct fix as it won't be properly compatible with concurrent mode, where the render phase (including the constructor) might be called more than once.

[gaearon]

Like Sophie says, this isn't the right fix. #63 (comment)