Microsoft/TypeScript

Typescript can't infer types when using Proxy

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#20,846 opened on Dec 21, 2017

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 (12 comments) (102 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: lib.d.tsHelp Wanted

Description

Code

let obj = {
    prop1: function () { },
    prop2: 'hello',
}

let prox = new Proxy(obj, {
    get: function (target, name) {
        return 5;
    }
});

prox.prop1.

Expected behavior: I would expect that when I type prox.prop1., I would get typescript suggestions for Number.prototype, but instead, I get suggestions for Function.prototype. prox.prop1 will (according to typescript) still be callable as a function, but in runtime, it will clearly be a number and will throw an exception.

Statically evaluate the proxy traps and determine the type of thing being returned to offer proper typescript intellisense.

Contributor guide

Tech stack
typescript
Domain
tooling
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.
needs investigation
Prerequisites
TypeScript type systemProxycompiler internalstype inference
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
This issue concerns TypeScript's inability to infer types through Proxy traps. The desired behavior is that the return type of a proxy get handler should override the original property type. A fix would likely involve modifying the type checker in src/compiler/checker.ts to handle Proxy declarations specially, possibly by analyzing the get trap's return type. The community discussion may contain further insights; see comments for potential approaches.