Properties with string keys and already declared type are getting their type narrowed when using square bracket assignment. #27412

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1. mikea commented on Sep 28, 2018

   

   Mike Aizatsky (mikea)

   on Sep 28, 2018

   Last edited by mikea

   More actions

   Another example of the code it breaks:

   interface IColor {
     r: number;
     g: number;
     b: number;
   }
   
   interface ITheme {
     primaryColor?: IColor;  
     "primary-color"?: IColor;
   }
   
   const red: IColor = { r: 1, g: 0, b: 0 };
   
   function fillInRedColor(theme: ITheme) {
     if (!theme.primaryColor) {
       theme.primaryColor = red;  
     }
     if (!theme["primary-color"]) {
       theme["primary-color"] = red;  // Type 'IColor' is not assignable to type 'undefined'
     }
   }

   http://www.typescriptlang.org/play/index.html#src=interface%20IColor%20%7B%0D%0A%20%20r%3A%20number%3B%0D%0A%20%20g%3A%20number%3B%0D%0A%20%20b%3A%20number%3B%0D%0A%7D%0D%0A%0D%0Ainterface%20ITheme%20%7B%0D%0A%20%20primaryColor%3F%3A%20IColor%3B%20%20%0D%0A%20%20%22primary-color%22%3F%3A%20IColor%3B%0D%0A%7D%0D%0A%0D%0Aconst%20red%3A%20IColor%20%3D%20%7B%20r%3A%201%2C%20g%3A%200%2C%20b%3A%200%20%7D%3B%0D%0A%0D%0Afunction%20fillInRedColor(theme%3A%20ITheme)%20%7B%0D%0A%20%20if%20(!theme.primaryColor)%20%7B%0D%0A%20%20%20%20theme.primaryColor%20%3D%20red%3B%20%20%0D%0A%20%20%7D%0D%0A%20%20if%20(!theme%5B%22primary-color%22%5D)%20%7B%0D%0A%20%20%20%20theme%5B%22primary-color%22%5D%20%3D%20red%3B%20%20%0D%0A%20%20%7D%0D%0A%7D%0D%0A%0D%0A
2. lazytype

   changed the title [-]Properties with string keys and already declared type are getting their type reduced when using square bracket assignment.[/-] [+]Properties with string keys and already declared type are getting their type narrowed when using square bracket assignment.[/+] on Sep 28, 2018
3. Anders Hejlsberg (ahejlsberg)

   assigned

   Nathan Shively-Sanders (sandersn)

   on Sep 28, 2018
4. Anders Hejlsberg (ahejlsberg)

   added

   BugA bug in TypeScriptA bug in TypeScript

   on Sep 28, 2018
5. Anders Hejlsberg (ahejlsberg)

   added this to the TypeScript 3.2 milestone on Sep 28, 2018
6. Nathan Shively-Sanders (sandersn)

   added a commit that references this issue on Oct 5, 2018

   Elt access assignment uses declared, not narrowed type

   ...

   This issue will close once commit f5190d1 is merged into the 'main' branch.

   f5190d1
7. Nathan Shively-Sanders (sandersn)

   closed this as completedin #27574on Oct 5, 2018
8. Nathan Shively-Sanders (sandersn)

   added a commit that references this issue on Oct 5, 2018

   Elt access assignment uses declared, not narrowed type (#27574)

   ...

   This issue will close once commit c080324 is merged into the 'main' branch.

   c080324
9. microsoft

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