Compiler complains about possible undefined in nested objects

Question

I have the following sample code: sample on typescriptlang.org (activate strictNullChecks)

 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. }

Why does the compiler complains about line 11.: Object is possible undefined when I explicitly check for this object in line 10.?

Can TypeScript be multi-threaded? If so, there's the possibility that objectVar[selector] could become undefined between the if check and the if block. I found that if I saved objectVar[selector] to a variable before the if statement, and referenced that variable instead, the error went away. — Alex Peters
– Alex Peters, Commented Sep 21, 2018 at 14:12
@AlexPeters TypeScript transpiles to JavaScript and a crucial aspect of JS VMs is that they are single-threaded. — Ahmed Fasih
– Ahmed Fasih, Commented Sep 25, 2018 at 14:48

Titian Cernicova-Dragomir · Accepted Answer · 2018-09-21 14:18:53Z

1

Type guards (which is what you are using when you write if (objectVar[selector]) and expect the type to change based on the check) do not work with index access. This is documented in this and this issues. The reason for this is (as stated by @RyanCavanaugh in comments to the issue)

Declined due to performance reasons. Since it should almost always be possible to write const j = list[i] instead, this shouldn't be too burdensome.

As is stated above the recommendation is to use a local variable :

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
}

edited Sep 21, 2018 at 14:18

answered Sep 21, 2018 at 14:13

Titian Cernicova-Dragomir

253k37 gold badges465 silver badges395 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

scipper Over a year ago

Very nice. good walkaround. thanks for pointing that out for me :)

Collectives™ on Stack Overflow

Compiler complains about possible undefined in nested objects

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related