It infers the type here:

let a: string | undefined;
let b = "";

if(typeof a === "string"){
  b = a
}

But it doesnt here:

let a: string | undefined;
let b: string | undefined;
let c = ""
if(typeof a === "string" || typeof b === "string"){
  c= a || b
}

Type 'string | undefined' is not assignable to type 'string'. Type 'undefined' is not assignable to type 'string'

Playground

What would be the correct way to write the code here ? Obviously, without unrolling the conditional.

CodePudding user response：

In this case, your code is incorrect and Typescript is correct. If a is the empty string and b is undefined, then the condition is true but c becomes undefined.

I would simply write it as follows:

let a: string | undefined;
let b: string | undefined;
const c = a ?? b ?? "";

Generally speaking Typescript does not work well with disjunctions of constraints, so better to avoid this kind of pattern.

CodePudding user response：

If you pay attention on the code you see that c need to be string and your condition can assign to c the undefined type.

let a: string | undefined;
let b: string | undefined;
let c: string | undefined = ""; // Now this var can be string or undefined

if(typeof a === "string" || typeof b === "string"){
    c = a || b
}