microsoft / microsoft/TypeScript

Inference for Object.isPrototypeOf()

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Description

Hi, I am wondering would be possible to infer types for isPrototypeOf() function same way as it's done with instanceof operator? So we could narrow type of argument if it's been created using Object.create(proto) function and its type would be something like typeof proto if check is fulfilled.

Search Terms

isPrototypeOf

Examples

if (object instanceof HTMLAnchorElement) {
    console.log(object.href);
}

if (HTMLAnchorElement.prototype.isPrototypeOf(object)) {
   console.log(object.href);
}

Checklist

My suggestion meets these guidelines:

  • This wouldn't be a breaking change in existing TypeScript/JavaScript code
  • This wouldn't change the runtime behavior of existing JavaScript code
  • This could be implemented without emitting different JS based on the types of the expressions
  • This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
  • This feature would agree with the rest of TypeScript's Design Goals.

Contributor guide

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First steps

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  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

No implementation file, test, or entry point is identified in the issue. Start by locating the existing type narrowing for instanceof, then evaluate how the Object.isPrototypeOf() examples should narrow objects created with Object.create(proto); done means the proposed behavior is specified and covered by appropriate compiler tests.

Written by the indexing model from the issue text.

Assessment

Tech stack
javascript, typescript
Domain
compilers
Issue type
Feature
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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