TypeScript shouldn't assume {[name: string]: NonUndefinedType} has no undefined properties #23817

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1. MartinJohns commented on May 2, 2018

   

   Martin Johns (MartinJohns)

   on May 2, 2018

   Last edited by MartinJohns

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   This is a known issue and is unfortunately "as intended", see #9235, #13161, #12287, and #7140. It's the same reason why accessing arrays by index does not result in a possibly undefined type, even tho theoretically the array could be empty. You'd need to add security checks basically at every indexer and array access, which would add a lot of bloat code to cases where usually you know beforehand that you have a value.

   A common practice is to leave it as it is when you know that you will always access known keys, or explicitly add undefined to your indexer type:

   interface Foo {
       [_: string]: number | undefined;
   }

   There's still an open proposal to add a compiler flag to automatically mark such indexers as undefined with an additional strict-flag: #13778
2. ghost added

   DuplicateAn existing issue was already createdAn existing issue was already created

   on May 2, 2018
3. typescript-bot commented on May 16, 2018

   

   TypeScript Bot (typescript-bot)

   on May 16, 2018

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   Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.
4. TypeScript Bot (typescript-bot)

   closed this as completedon May 16, 2018
5. microsoft

   locked and limited conversation to collaborators on Jul 31, 2018