setState vs replaceState in React.js

Question

I am new to React.js Library and I was going over some of the tutorials and I came across:

• this.setState
• this.replaceState

The Description given is not very clear (IMO).

setState is done to 'set' the state of a value, even if its already set 
in the 'getInitialState' function.

Similarly,

The replaceState() method is for when you want to clear out the values 
already in state, and add new ones.

I tried this.setState({data: someArray}); followed by this.replaceState({test: someArray}); and then console.logged them and I found that state now had both data and test.

Then, I tried this.setState({data: someArray}); followed by this.setState({test: someArray}); and then console.logged them and I found that state again had both data and test.

So, what exactly is the difference between the two ?

Answer 1

With setState the current and previous states are merged. With replaceState, it throws out the current state, and replaces it with only what you provide. Usually setState is used unless you really need to remove keys for some reason; but setting them to false/null is usually a more explicit tactic.

While it's possible it could change; replaceState currently uses the object passed as the state, i.e. replaceState(x), and once it's set this.state === x. This is a little lighter than setState, so it could be used as an optimization if thousands of components are setting their states frequently.
_{I asserted this with this test case.}

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If your current state is {a: 1}, and you call this.setState({b: 2}); when the state is applied, it will be {a: 1, b: 2}. If you called this.replaceState({b: 2}) your state would be {b: 2}.

Side note: State isn't set instantly, so don't do this.setState({b: 1}); console.log(this.state) when testing.