Microsoft/TypeScript

`.length` property of function literals is not `readonly`

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#49,113 opened on May 14, 2022

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BugDomain: lib.d.tsHelp Wanted

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Description

Bug Report

Function .length property is inconsistently readonly depending on whether Function constructor is used or not.

🔎 Search Terms

function length readonly

🕗 Version & Regression Information

v4.6.2 (haven't checked older versions)

⏯ Playground Link

Playground link with relevant code

💻 Code

const f = new Function()
f.length = 1 // Error here: Cannot assign to 'length' because it is a read-only property.

const f2 = () => {}
f2.length = 1 // No error here

🙁 Actual behavior

The length property of the function created with new Function() is readonly while the length property of function literals is not.

🙂 Expected behavior

Both length properties should be readonly. Alternative both could be non-readonly but that seems worse since JS doesn't modify the length when assigning a value.

Contributor guide

Research direction
Examine the type definitions for function literals in TypeScript's checker source code to find where the 'length' property is not marked readonly, and compare with the Function constructor type to understand the inconsistency.
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.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScriptGit
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65