Typescript: Variable being used before assigned – isn't that correct?

Why is the following code invalid in strict mode:

let x: string[];
if (Math.random() > .5){ x = ['x']; }

//TS2454: x possible undefined. Yeah, I know?
return x?.map(v => v);

Yeah, I know that x may be undefined. But that’s why I’m using the Optional Chaining Operator. And that sure perfectly works on undefined.

>Solution :

The code is invalid in strict mode because TypeScript’s strict mode includes the strictNullChecks option, which ensures that variables cannot be used before they are assigned.

In your code, x is declared but not necessarily initialized before it is used in the return statement.

You have then 3 options

1. Initialize x with a default value

let x: string[] = [];

2. Use a Type Union with undefined

let x: string[] | undefined;

3. Add an Else Case to Initialize x

let x: string[];
if (Math.random() > .5) {
   x = ['x'];
} else {
   x = [];
}

return x.map(v => v);