What is the difference between using constructor vs state = {} to declare state in react component?

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They are roughly equivalent. The significant difference is that the initializer in the second example is executed before constructor.

The second approach uses class fields proposal.

It is not a part of ECMAScript standard yet so you need to set up transpiler properly to use the second approach.

UPD Take a look at Babel output to better understand how public instance fields work.

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