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Why are my React functions running twice?


    John Partee

I'm a fairly new React developer, I've only been working on it for about the last two years, very off and on. I've noticed some effects being called more than once, but never really thought much of it. With Tacticle I had to use a mutable object (a chess game, thanks chess.js!) in my state, which caused real problems when I was modifying it.

Sources and Problem

Thanks to this stackoverflow thread and this github issue, I figured out why. React is designed to help prevent side effects in code, and functions are run twice on purpose only in development to try to emulate what could happen if a page was disconnected and reconnected, or some other bug that caused a function to actually run twice. In the perfect functional world, nothing would happen. But with mutable state, this is a problem!


create-react-app - The boilerplate that a lot of people use, enables React strict mode by default. In our index.jsx (or .tsx!), we'll see something like this:


    <App />

Just remove the <React.StrictMode> component!


ReactDOM.render(<App />, document.getElementById('root'))

Be Warned!

React.StrictMode is there to help prevent real problems in your code while developing. It shouldn't cause functions to double run in production. In this case, I couldn't find a way around mutating my state directly, so I had to disable strictmode, which comes with bug risks.

React Documentation covers what StrictMode does - Check it out if you haven't read up on it before.

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