Microsoft/TypeScript

Differing user-defined type guard and 'typeof' type guard behaviour when narrowing 'any'

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#6,015 opened on Dec 9, 2015

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 (11 comments) (0 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
Effort: ModerateHelp WantedSuggestion

Description

In the below code, using a typeof type guard and an equivalent (I thought) user-defined guard, only one error is produced.

var y: any;

// Built-in type guard
if (typeof y === "string") {
    y.hello = true; // Correct error - 'hello' does not exist on type string
}

// Equivalent user-defined type guard
function f(x: any): x is string {
    return typeof x === "string";
}

if (f(y)) {
    y.hello = true; // No error with user-defined guard
}

Playground demo. Looks like user-defined type guards won't narrow from any, in any circumstances, as far as I can tell.

Contributor guide

Tech stack
typescript
Domain
developer experience
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Understanding TypeScript type narrowingFamiliarity with TypeScript compiler architecture
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
Investigate the TypeScript compiler source code, particularly the `narrowTypeByTypeOf` and `narrowTypeByExpression` functions in `src/compiler/checker.ts`. The issue is that user defined type guards do not narrow `any` to the guarded type, while `typeof` guards do. Review the logic that applies type predicates to see why `any` is not narrowed. There is a discussion in the comments suggesting that the predicate's type guard might be ignored when the input is `any`. Consider whether the fix should be in the narrowing logic or in the predicate resolution.