No error for missing method call  #35557

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1. Matt Bierner (mjbvz)

   changed the title [-]No error for missing method call if other property is accessed in conditional body[/-] [+]No error for missing method call [/+] on Dec 6, 2019
2. MartinJohns commented on Dec 7, 2019

   

   Martin Johns (MartinJohns)

   on Dec 7, 2019

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   This will only report an error if strictNullChecks is enabled, and stats.isDirectory is a non-nullable function. See #33178 and #9041.
3. mjbvz commented on Dec 7, 2019

   

   Matt Bierner (mjbvz)

   on Dec 7, 2019

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   This is in strict mode:

   
4. Ryan Cavanaugh (RyanCavanaugh)

   added

   BugA bug in TypeScriptA bug in TypeScript

   on Dec 9, 2019
5. Ryan Cavanaugh (RyanCavanaugh)

   added this to the TypeScript 3.8.1 milestone on Dec 9, 2019
6. RyanCavanaugh commented on Dec 9, 2019

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Dec 9, 2019

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   It looks like we're confused by any reference to stats in the body if the if
7. dragomirtitian commented on Dec 26, 2019

   

   Titian Cernicova-Dragomir (dragomirtitian)

   on Dec 26, 2019

   Last edited by dragomirtitian

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   The way the check is done for usage seems brittle. The only check that is done is for symbol id equality:

   const functionIsUsedInBody = forEachChild(ifStatement.thenStatement, function check(childNode): boolean | undefined {
       if (isIdentifier(childNode)) {
           const childSymbol = getSymbolAtLocation(childNode);
           if (childSymbol && childSymbol.id === testedFunctionSymbol.id) { // HERE 
               return true;
           }
       }
   
       return forEachChild(childNode, check);
   });

   This has the drawback of not actually working at all for symbols which have not had an id assigned (and we get undefined === undefined). This could be fixed by using getSymbolId which would assign the id, or by direct object equality (since as far as I understand it the same symbol object should be used and id equality when assigned at this location seems no better) but this fix would not fix the following surprising behavior:

   interface StatsBase {
       isDirectory(): boolean;
   }
   function A(stats2: StatsBase, stats: StatsBase) {
       if (stats.isDirectory) { // no error because we access isDirectory on stat2
           stats2.isDirectory(); // comment this and we get an error above
       }
   }

   Playground Link

   The symbols isDirectory is used, in the if condition and in the body but the target is different.

   Proposed solution:

   1. Use getSymbolId for when comparing symbol ids
   2. Compare the expression of the property access to ensure it is the same (if the testedNode is a property access)
8. Ryan Cavanaugh (RyanCavanaugh)

   modified the milestones: TypeScript 3.8.1, Backlog on Jan 24, 2020
9. Nathan Shively-Sanders (sandersn)

   closed this as completedin #35862on Mar 30, 2020
10. microsoft

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