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Type Gaurds Don't Narrow Type in Inner Functions #10927

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Type Gaurds Don't Narrow Type in Inner Functions#10927
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QuestionAn issue which isn't directly actionable in code

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@ethanresnick
Ethan Resnick (ethanresnick)
opened on Sep 14, 2016

TypeScript Version: 2.0.2

Code

  function isNumber(test: number|string): test is number {
    return typeof test === "number";
  }
  function blah(arg: () => void): void { }

  let x: any = 4;

  if(isNumber(x)) {
    x; // x is correctly narrowed to number here
    blah(() => {
      x; // typeof x is any again
    })
  }

Expected behavior:
(I think) x should still be inferred as type number in the inner function.

Actual behavior:
x is inferred as any

Activity

  1. normalser commented on Sep 14, 2016

    @normalser
    normalser
    on Sep 14, 2016

    #7719 (comment)

  2. ahejlsberg commented on Sep 14, 2016

    @ahejlsberg
    Anders Hejlsberg (ahejlsberg)
    on Sep 14, 2016
    Member

    FYI, it works as expected if you use const instead of let because the compiler then knows that x will never be reassigned.

  3. Mohamed Hegazy (mhegazy)
    added
    QuestionAn issue which isn't directly actionable in code
    on Sep 14, 2016
  4. Mohamed Hegazy (mhegazy)
    closed this as completedon Sep 14, 2016

  5. ethanresnick commented on Sep 15, 2016

    @ethanresnick
    Ethan Resnick (ethanresnick)
    on Sep 15, 2016
    ContributorAuthor

    Got it. Thanks!

  6. microsoft
    locked and limited conversation to collaborators on Jun 19, 2018
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