Erroneous type narrowing in finally #18478

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1. Mohamed Hegazy (mhegazy)

   added

   BugA bug in TypeScriptA bug in TypeScript

   on Sep 14, 2017
2. Mohamed Hegazy (mhegazy)

   assigned

   Nathan Shively-Sanders (sandersn)

   on Sep 14, 2017
4. Mohamed Hegazy (mhegazy)

   added this to the TypeScript 2.9 milestone on Apr 12, 2018
5. Mohamed Hegazy (mhegazy)

   modified the milestones: TypeScript 2.9, TypeScript 3.0 on Apr 26, 2018
6. Mohamed Hegazy (mhegazy)

   modified the milestones: TypeScript 3.0, Future on Jul 2, 2018
7. rdsedmundo commented on Feb 6, 2019

   

   Edmundo Santos (rdsedmundo)

   on Feb 6, 2019

   More actions

   Also facing this while setup a DB connection and trying to finalize it under the finally block if it was created.

     let connection;
   
     try {
       connection = await getConnection();
   
       const { rows } = await connection.query(sql, values);
   
       return camelizeKeys(rows);
     } finally {
       if (connection) {
         // [ts] Property 'end' does not exist on type 'never'. [2339]
         await connection.end();
       }
     }

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8. dtiziani commented on Dec 12, 2019

   

   Daniel Tiziani (dtiziani)

   on Dec 12, 2019

   Last edited by dtiziani

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   I'm facing the SAME issue, trying to finalize a db connection.

   What I could figure out is that it happends when a function returns anything at all, it presents the error, but if it's a void function, the issue is not present.

   Here's a playground with both scenarios: http://www.typescriptlang.org/play/?ssl=1&ssc=1&pln=29&pc=4#code/FAMwrgdgxgLglgewgAhHCATAQgTwHICGAtgKYAUExJAXMgM4wBO6A5gJS0BuCcGyA3sGTDkAGxIxkUJBBKxEEWg2YQWyAD7IIYUaOQBeLTtEBuYEJFMcAiyKky58JAa1UzdgL5SCMKAAtkMhI2Gzs7aQg6BHEAOlEEFiC2WzCYP0YEAHdkEncRLzRKXWtBMOQ4EDII2XlnAEJDbV0QiKjY+MTqxwU4klU05M8LD3NQSFqUQuwcAEkMMl4lJlYOemXVDSNdUJFxSS6JpZU1TSa9RuMzFKsd8IcJl14rsuFGCTBGFAOnCDzhLygPn8gWCtzCrWiJDiCSSKTsaQy2VyKQK6AIxTBwgqVXuP2QDS2ohaSDaUI6OIgNR+vX6fkG+WGQA

   also the code below:

   
   // No issues at all
   function findByName(name: string): void {
       let connection: string | null = null;
   
       try {
         connection = name;
       } catch (e) {
           console.log(e)
           throw e;
       } finally {
         if(connection != null) console.log(connection.length)
       }
     }
   
   // variable connection as never on finally block
   function findById(id: string): string | null {
       let connection: string | null = null;
   
       try {
         connection = id;
   
           return connection;
       } catch (e) {
           console.log(e)
           throw e;
       } finally {
         if(connection != null) console.log(connection.length)
       }
     }
   
   

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9. Nathan Shively-Sanders (sandersn)

   removed their assignment

   on Jan 7, 2020
10. SlurpTheo commented on Jan 9, 2020

    

    SlurpTheo

    on Jan 9, 2020

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    I can't reproduce the earlier problems in this issue anymore (using typescript@3.7.4), but I do get the same CFA problem described by Daniel Tiziani (@dtiziani) in the findById()-example.

    Commenting-out either the return or the throw statement prevents the problem.

    ---

    Now, if you slightly tweak the example (to use undefined instead of null), there's another interesting bit that not explicitly initializing to = undefined prevents the problem as well:

    function findById2(id: string) {
       // not explicitly initializing this to `= undefined` prevents the problem
       let connection: string | undefined = undefined;
    
       try {
          connection = id;
    
          // commenting-out this `return` -OR- the `throw` below prevents the problem
          return connection;
       } catch (e) {
          console.log(e);
    
          // commenting-out this `throw` -OR- the `return` above prevents the problem
          throw e;
       } finally {
          if (connection) {
             // ts(2339) -
             //   Property 'length' does not exist on type 'never'.
             console.log(connection.length);
          }
       }
    }

    And this "work-around" reminds me of this CFA conversation... only there's no function expression involved.
11. RyanCavanaugh commented on Jan 9, 2020

    

    Ryan Cavanaugh (RyanCavanaugh)

    on Jan 9, 2020

    Member

    More actions

    SlurpTheo fixed in nightly; see #34880

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12. Ryan Cavanaugh (RyanCavanaugh)

    added

    FixedA PR has been merged for this issueA PR has been merged for this issue

    on Jan 9, 2020
13. Ryan Cavanaugh (RyanCavanaugh)

    assigned

    Anders Hejlsberg (ahejlsberg)

    on Jan 9, 2020
14. Ryan Cavanaugh (RyanCavanaugh)

    closed this as completedon Jan 9, 2020
15. microsoft

    locked as resolved and limited conversation to collaborators on Oct 21, 2025