Microsoft/TypeScript

Typescript supports less valid variable names characters than javascript

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#42,452 opened on Jan 22, 2021

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 (1 comment) (1 reaction) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: ParserHelp Wanted

Description

Bug Report

🔎 Search Terms

unicode invalid character name

🕗 Version & Regression Information

  • I specifically tried the zero width space non-joiner. In this particular case at least, the support was lost between versions 3.5.1 and 3.6.3, as shown by the playground.

⏯ Playground Link

Playground link with relevant code

💻 Code

// There is a zero width non-joiner between "x" and "y".
// All versions starting from 3.6.3 think that I am defining a "x" and a "y" variable separately.
// Version 3.5.1 outputs the expected JS code, which is strictly equal to the TS code in this case.
const x‌y = 42;

🙁 Actual behavior

Not all variable names valid in JS are supported in TS. At least the zero width non-joiner character is not supported. I didn't go on the hunt for all the unsupported characters but clearly TS does not support the full set of supported JS characters anymore.

🙂 Expected behavior

All valid variable names in JS should also be valid variable names in TS. At least when tsconfig is configured to output in an ES version that supports it.

For the record, a wonderful tool exist to test if a variable name is valid in JS: https://mothereff.in/js-variables

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.
3
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.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
TypeScript compiler build setupKnowledge of JavaScript identifier specification
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
The bug is that TypeScript does not accept all valid JavaScript identifier characters, specifically the zero width non joiner. The issue likely lies in the scanner/parser logic that validates identifiers in src/compiler/scanner.ts. Investigate the character validation functions (e.g., isIdentifierStart, isIdentifierPart) and compare with the JavaScript specification. Check the changes between TypeScript versions 3.5.1 and 3.6.3 to identify the regression. The playground link demonstrates the problem; test with various Unicode characters to determine the full set of unsupported characters.