Microsoft/TypeScript

Type Inference in for... in combined with typeof

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#12,944 opened on Dec 15, 2016

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Description

TypeScript Version: 2.1.4

The following code gives an error in TS 2.1: It's not really nice code, but I think it should work never the less :)

for (var a in this) {
            if (typeof this[a] === "function" && !ko.isObservable(this[a]))
                this[a] = this[a].bind(this);
        }

Expected behavior: I would expect this[a] to be typed as this[keyof this] & Function:

for (var a in this) {
            if (typeof this[a] === "function" && !ko.isObservable(this[a]))
                this[a] =( <this[keyof this] & Function> this[a]).bind(this);
        }

Actual behavior: It gives an error, as of TS 2.1, it worked in TS 2.0 (because mapped types did not exist, this[a] was typed as any)

Contributor guide

Research direction
Investigate how the type checker narrows the type of `this[a]` inside a `for...in` loop when using `typeof` and adjust the control flow analysis to properly intersect the narrowed type with `Function`.
Tech stack
typescript
Domain
compilers
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
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
TypeScripttype inference
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30