Incorrect "did you mean to call" error issued for invoked class property #38161

Activity

1. Ryan Cavanaugh (RyanCavanaugh)

   added

   BugA bug in TypeScriptA bug in TypeScript

   on Apr 24, 2020
2. Ryan Cavanaugh (RyanCavanaugh)

   added this to the TypeScript 3.9.1 milestone on Apr 24, 2020
3. Ryan Cavanaugh (RyanCavanaugh)

   assigned

   Andrew Branch (andrewbranch)

   on Apr 24, 2020
4. Andrew Branch (andrewbranch)

   closed this as completedin #38163on Apr 24, 2020
5. decherneyge commented on Jun 17, 2020

   

   George DeCherney (decherneyge)

   on Jun 17, 2020

   More actions

   im not 100% sure this is the same issue but I thought I would check here first before opening a separate issue.

   This image is from the typescript playground. Im adding it to show that the compiler is barking at the null check if/when the property is not used within the block but not when it is used which seems inconsistent. The error in both places shown with a red underline is "This condition will always return true since the function is always defined. Did you mean to call it instead?(2774)"
   

   

   Code:

   class Widget {
       public refresh: (() => void);
       constructor() {
           this.refresh = () => { };
       }
   }
   
   class Main{
   
       public widgets: Array<Widget> = [];
       public $timeout: any;
   
       constructor() {
   
           const filterable: Array<Widget> = this.widgets.filter((x) => {
               //error here
               if (x.refresh) {
                   //x.refresh();
               }
               //no error here
               if (x.refresh) {
                   x.refresh();
               }
           });
           
       }
   
       private refreshWidget(w: Widget) {
           //error here
           if (w.refresh) {
               this.$timeout(() => {
                   //w.refresh();
                });
           }
           //no error here
           if (w.refresh) {
               this.$timeout(() => {
                   w.refresh();
                });
           }
   
       }
   
   }
    
   
6. RyanCavanaugh commented on Jun 17, 2020

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Jun 17, 2020

   MemberAuthor

   More actions

   George DeCherney (@decherneyge) that is precisely the intended behavior
7. decherneyge commented on Jun 17, 2020

   

   George DeCherney (decherneyge)

   on Jun 17, 2020

   More actions

   Ryan Cavanaugh (@RyanCavanaugh) oh ok. Thanks for the reply! Why is it the intended behavior?
8. RyanCavanaugh commented on Jun 17, 2020

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Jun 17, 2020

   MemberAuthor

   More actions

   George DeCherney (@decherneyge) See discussion in #32802 and linked issues from there

   👍React with 👍1Reacted by George DeCherney
9. luryson commented on Nov 17, 2021

   

   luryson

   on Nov 17, 2021

   Last edited by luryson

   More actions

       interface A {
           a: () => boolean;
       }
   
       class AImpl implements A {
           public a(): boolean {
               return true;
           }
       }
   
       it('should fail', function () {
           const aImpl = new AImpl();
           if (aImpl.a) { // TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
               console.log("always true");
           }
       });
   
       it('why success', function () {
           const aImpl = new AImpl();
           if (!aImpl.a) { // nothing happens here
               console.log('always false');
           }
       });

   just wondering why !aIpl.a not barking TS2774
10. microsoft

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