Compiler raises concerns about potential undefined values in nested objects

In my code snippet, I am experiencing an issue with TypeScript when I try to access an object property after checking for its existence. The sample code can be found here (when strictNullChecks is enabled).

 1. let boolVar: number | undefined;
 2. 
 3. if (boolVar) {
 4.     boolVar.toString; // all fine
 5. }
 6. 
 7. let objectVar: { [key: string]: number | undefined } = {a: 1};
 8. 
 9. const selector = "a";
10. if (objectVar[selector]) {
11.     objectVar[selector].toFixed; // Object is possible undefined? o0
12. }

I am confused as to why the TypeScript compiler is throwing the error "Object is possible undefined" on line 11, considering I have explicitly checked for the object's existence in line 10. Can anyone help me understand this issue?

typescript

Answer №1

When writing code such as if (objectVar[selector]) and expecting the type to change based on the check, it's important to note that type guards, which are in use here, do not function with index access. This limitation has been documented in both this and this issues. According to comments on the issue by @RyanCavanaugh:

The use of type guards with index access has been declined due to performance reasons. The recommended approach is to use a local variable instead, like const j = list[i], which should not pose a significant burden.

Therefore, the advice is to utilize a local variable as shown below:

let objectVar: { [key: string]: number | undefined } = {a: 1};

const selector = "a";
const objectVar_selector = objectVar[selector]; 
if (objectVar_selector) {
    objectVar_selector.toFixed; // Object is possible undefined? o0
}

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