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asserts keyword does not enhance hoisted function type, works on functions assigned to a variable #41232

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phryneas opened this issue Oct 25, 2020 · 2 comments · May be fixed by #58411
Open

asserts keyword does not enhance hoisted function type, works on functions assigned to a variable #41232

phryneas opened this issue Oct 25, 2020 · 2 comments · May be fixed by #58411
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Bug A bug in TypeScript Help Wanted You can do this
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Backlog

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@phryneas
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phryneas commented Oct 25, 2020 •
edited

TypeScript Version: 4.0.2, 4.1.0-beta

Search Terms:
asserts function

Code

interface LabelledFunction { label: string; }

function assignLabel(fn: () => unknown, label: string): asserts fn is LabelledFunction {
  Object.assign(fn, { label });
}

function a() { }
assignLabel(a, 'a');
console.info(a.label); // Property 'label' does not exist on type '() => void'.(2339)

const b = function () { };
assignLabel(b, 'b');
console.info(b.label); // works as expected

const c = () => { };
assignLabel(c, 'c');
console.info(c.label); // works as expected

Expected behavior:

At least in runtime code coming after assignLabel(a, 'a');, a.label should be accessible.

Actual behavior:

Property 'label' does not exist on type '() => void'.(2339)

Playground Link: link
@phryneas phryneas changed the title asserts keyword does not enhance hoisting function type, works on functions assigned to a variable asserts keyword does not enhance hoisted function type, works on functions assigned to a variable Oct 25, 2020
@robbiespeed
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Same thing happens to classes:

interface Labelled {
  label: string;
}

function assignLabel <T> (value: T, label: string): asserts value is (T & Labelled) {
  Object.assign(value, { label });
}

function a() { }
assignLabel(a, 'a');
console.info(a.label); // Property 'label' does not exist on type '() => void'.(2339)

const b = function b () { };
assignLabel(b, 'b');
console.info(b.label); // Works

class X {}
assignLabel(X, 'X');
X.label; // Property 'label' does not exist on type '() => void'.(2339)

const Y = class Y {}
assignLabel(Y, 'Y');  // Works
Y.label;

@RyanCavanaugh RyanCavanaugh added Bug A bug in TypeScript Help Wanted You can do this labels Nov 4, 2020
@RyanCavanaugh RyanCavanaugh added this to the Backlog milestone Nov 4, 2020
@denismaxim0v
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I think I could look into it

@MartinJohns MartinJohns mentioned this issue Aug 19, 2022
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@Andarist Andarist linked a pull request May 2, 2024 that will close this issue
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Compute flow types for function declarations Andarist/TypeScript
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@robbiespeed @phryneas @RyanCavanaugh @denismaxim0v