TypeScript complains about unassigned variable despite exhaustive if/if else statement-CodePudding

Consider this code snippet:

type MyType = "value1" | "value2"

function myFunction(myParam: MyType) {
  let myString: string
  if(myParam === "value1") {
    myString = "foo1"
  }
  else if(myParam === "value2") {
    myString = "foo2"
  }
  myOtherFunction(myString) 
}

function myOtherFunction(anotherParam: string) { ... }

Since the if/if else handles all values of myParam, there is no way myString can be unassigned. Still, TypeScript complains: Variable 'myString' is used before being assigned.

How can I get rid of that error without substantial code changes?

(Of course, I could remove the if(myParam === "value2") part, but that makes the code much less readable. A switch works as well I think. But I dislike those.)

CodePudding user response：

The way to "fix" this is to throw in an additional else

type MyType = "value1" | "value2"

function myFunction(myParam: MyType) {
  let myString: string
  if(myParam === "value1") {
    myString = "foo1"
  }
  else if(myParam === "value2") {
    myString = "foo2"
  }
  else {
    // const unknownType = myParam; // You can use this line to check what type is myParam
    throw new Error('myParam is wrong!');
  }

  // ...
}

By throwing in the else you ensure typescript that the path that should not be taken really cannot be taken.

Also for this kind of thing, I tend to use switch for clarity, but it is viable only for primitives...

CodePudding user response：

The if .. else is exhaustive only when the last else does not have if:

type MyType = "value1" | "value2"

function myFunction(myParam: MyType) {
  let myString: string
  if(myParam === "value1") {
    myString = "foo1"
  }
  else { // <-- no "if" here
    myString = "foo2"
  }

  // `myString` is definitely assigned a value here

  myOtherFunction(myString)
}

function myOtherFunction(anotherParam: string) { ... }

CodePudding user response：

You could switch to switch ^{(ugh.., sorry)} and make the second case default:

type MyType = "value1" | "value2"

function myFunction(myParam: MyType) {
  let myString: string

  switch (myParam) {
    case "value1":
      myString = "foo1"
      break
    case "value2":
    default:
      myString = "foo2"
      break
  }

  myOtherFunction(myString)
}

function myOtherFunction(anotherParam: string) { ... }