CODEWITHSUNDEEP
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Ikaer
Ikaer

Reputation: 37

Compilation error when using a variable initialized by an enum value inside a switch

Playground Link

I'm having a compilation error with the code provided in the playground link and I don't understand why.

I'm creating a variable with a value of an enum:

enter image description here

intellisense tell me that the type of my variable is inferred as I expected: ValidatorMessageState. Like when assigning true/false to a variable, typescript infers a boolean and not true or false as type.

then I switch over it: enter image description here Now intellisense tells me the type of my variable has changed even if I've done nothing between creating it and switching on it. Variable have now the type of the value of the enum and not the enum.

Another thing I've found strange: if I introduce some closure, typescript does not give me errors (I've commented this part in the playground)

enter image description here

I suppose I'm doing something wrong here, but I don't see what. Can someone could explain it to me?

Updated playground: Playground Link

Upvotes: 0

Views: 107

Answers (1)

Aviad Hadad
Aviad Hadad

Reputation: 1717

It's a feature, not a bug :)

This is Typescript doing control flow analysis. The control flow analysis only start kicking in in certain code structures, like a switch statement.

Once you reach the switch statement, typescript compiler tries to narrow down the type of maxStateEncoutered. And in your case it can only be ValidatorMessageState.Error as that was the only assignement. And typescript isn't wrong about this... If you assigned your variable in some dynamic way (as in: a function that returns ValidatorMessageState) the error would go away.

As for the code inside the forEach, typescript can't reason about code inside closures/callbacks. The gist of the reason being it doesn't know when this code will run. Sure, in forEach it will run immediately, but typescript doesn't know that about the function. In a function like addEventListener the function you pass can run at any time, and maxStateEncoutered might change until then, so it can no longer deduce that maxStateEncoutered is of specific type ValidatorMessageState.Error. If you change maxStateEncoutered from let to const, you will see this error inside the forEach as well.

Upvotes: 1

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