Correct setState() usage in tutorial

[ludofischer]

If the new state depends on the previous state, if I remember correctly, it’s safer to use setState() with a function argument to ensure we’re not reading from an outdated state.

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The tutorial suggests to use setState() with an object argument when the new state depends on the previous state. In such situations, it’s preferable to use a function to ensure the previous state is up-to-date.

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Updates documentation only, so there are no additional tests. Rendering the site.

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[lumiasaki]

If my memory is correct, get value from this.state, modify it, then setState() with new value of state is safe in early version of React Native, but it is not safe anymore in the latest version of React Native, thanks for your updating.