Incorrect error of use of unassigned variable #13811

Activity

1. devdoomari commented on Feb 1, 2017

   

   devdoomari (devdoomari)

   on Feb 1, 2017

   Last edited by devdoomari

   More actions

   a has the type of {}, and has no value assigned.
   i.e. your code is equivalent to:

   let a: {} = undefined;
   

   so what you wanted:

   let a = {}; // not let a: {}
   
2. NN--- commented on Feb 1, 2017

   

   NN (NN---)

   on Feb 1, 2017

   Last edited by NN---

   Author

   More actions

   devdoomari (@devdoomari) It is not what I want.
   I want to have it undefined and assign it only if it is undefined.

   Full code:

   let a: {};
   
   for (const x of y) {
     if(x.q) {
       a =x.q;
       break;
     }
   }
   
   // Assign default value if couldn't assign before
   if (!a) {
     a = {};
   }
3. ahejlsberg commented on Feb 1, 2017

   

   Anders Hejlsberg (ahejlsberg)

   on Feb 1, 2017

   Member

   More actions

   The control flow checker (in strict null checks mode) assures that you'll never accidentally observe undefined in a variable unless the type of the variable allows it. Your program logic is constructed such that undefined is actually a permitted value (at least in parts of the logic), so I'd recommend including undefined in the type of a:

   let a: {} | undefined;

   The problem with your suggestion is that we'd no longer catch this:

   let a: string;
   // Intended to have assignment here, but forgot
   if (!a) {
       // Think I have an empty string, but I have undefined
   }
4. Anders Hejlsberg (ahejlsberg)

   added

   Working as IntendedThe behavior described is the intended behavior; this is not a bugThe behavior described is the intended behavior; this is not a bug

   on Feb 1, 2017
5. NN--- commented on Feb 1, 2017

   

   NN (NN---)

   on Feb 1, 2017

   Author

   More actions

   Anders Hejlsberg (@ahejlsberg) What about this case ?

   I would expect it compiling fine since I am checking against undefined.

   let a: number;
   if (typeof a !== "undefined") {
   	a = 1;
   }
6. ahejlsberg commented on Feb 1, 2017

   

   Anders Hejlsberg (ahejlsberg)

   on Feb 1, 2017

   Last edited by ahejlsberg

   Member

   More actions

   I think the same argument holds. If the declared type of a variable does not include undefined, the control flow analyzer takes it to mean you never want to observe the variable in an uninitialized state (in any way). I think the consistency of that is important, particularly since you can just add undefined to the type in cases where you want to do those types of checks.

   👍React with 👍2Reacted by Aluan Haddad and Kevin Ryan
7. NN--- commented on Feb 2, 2017

   

   NN (NN---)

   on Feb 2, 2017

   Author

   More actions

   That's what I ended with.
   Thanks.
8. NN--- commented on Feb 6, 2017

   

   NN (NN---)

   on Feb 6, 2017

   Author

   More actions

   Anders Hejlsberg (@ahejlsberg)
   I feel that there is a need for language construct or helper which tests for null or undefined.
   Currently I use "if(a) {..}" but if type of 'a' changes to string or number, this test will not be correct.

   I feel more safe using this helper now.

   function isNullOrUndefined<T>(obj: T | null | undefined): obj is null | undefined {
   	return typeof obj === "undefined" || obj === null;
   }
   
   if (!isNullOrUndefined(a)) { ... }

   But prefer to have something in the language like:

   iff (a) { ... }
9. ahejlsberg commented on Feb 6, 2017

   

   Anders Hejlsberg (ahejlsberg)

   on Feb 6, 2017

   Member

   More actions

   Adding entirely new statements beyond what JavaScript has is definitely out of scope for TypeScript. To check if a value is null or undefined in JavaScript, just use x == null. See more discussion here.
10. NN--- commented on Feb 6, 2017

    

    NN (NN---)

    on Feb 6, 2017

    Author

    More actions

    Thanks.
    Didn't think about this pattern
11. Mohamed Hegazy (mhegazy)

    closed this as completedon Apr 19, 2017
12. microsoft

    locked and limited conversation to collaborators on Jun 19, 2018