Search code examples
typescript

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[] = [];
    1. Use a Type Union with undefined
    let x: string[] | undefined;
    1. Add an Else Case to Initialize x
    let x: string[];
if (Math.random() > .5) {
   x = ['x'];
} else {
   x = [];
}

return x.map(v => v);