Hacktoberfest 2026: the issues maintainers tagged for October, open and beginner-friendly. Browse Hacktoberfest issues

[JavaScript] Tokenization breaks when new line starts with less-than operator

Open
#904 0 comments 0 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
javascript, typescript
Domain
tooling

Research direction

Start by reproducing the JavaScript example in VS Code with extensions disabled, then inspect the repository's TypeScript grammar files. The fix is done when a less-than operator at the start of a line is highlighted as an operator rather than opening JSX, and highlighting does not continue until a closing tag.

Written by the indexing model from the issue text.

Description

Does this issue occur when all extensions are disabled?: Yes

VS Code version: Code 1.66.0 (e18005f0f1b33c29e81d732535d8c0e47cafb0b5, 2022-03-30T05:50:41.156Z)
OS version: Linux x64 4.15.0-175-generic
Restricted Mode: No

System Info
Item Value
CPUs Intel(R) Core(TM) i7-7820HQ CPU @ 2.90GHz (8 x 3541)
GPU Status 2d_canvas: enabled
canvas_oop_rasterization: disabled_off
direct_rendering_display_compositor: disabled_off_ok
gpu_compositing: enabled
multiple_raster_threads: enabled_on
oop_rasterization: disabled_off
opengl: enabled_on
rasterization: disabled_software
raw_draw: disabled_off_ok
skia_renderer: enabled_on
video_decode: disabled_software
video_encode: disabled_software
vulkan: disabled_off
webgl: enabled
webgl2: enabled
Load (avg) 0, 0, 1
Memory (System) 31.26GB (23.84GB free)
Process Argv --unity-launch --crash-reporter-id c990702e-9bfc-48d9-b2b9-78bab7fdb823
Screen Reader no
VM 0%
DESKTOP_SESSION ubuntu
XDG_CURRENT_DESKTOP Unity
XDG_SESSION_DESKTOP ubuntu

|XDG_SESSION_TYPE|x11|

Extensions: none

Simple example

  1. In a new editor, set the language mode to plain JavaScript
  2. Type the following 
    let obj = {}

let bad = 1
  < obj.two

let unexpected = 1
/>
let nowFixed = 1
  3. The < on line 4 is treated as a JSX tag opener (even when the self-closing tag on line 7 is removed)
    Note how GitHub's highlighting in 2. and this screenshot disagree
    Screenshot from 2022-04-01 11-22-56
  4. Declaring two slightly changes highlighting, but notice the . in obj.two is still green. Using "Developer: Inspect Editor Token and Scopes" shows it's still considered a JSX tag.
    Screenshot from 2022-04-01 11-23-48

Extended example

To show all the specific conditions required to trigger this issue

Screenshot from 2022-04-01 11-17-14

Show code snippet for copy-paste 

let expected, notExpected, obj = {}
let expectedFunc = () => { return (true) }
obj.twenty = 20

expected = 10 < obj.twenty

expected = 10
  < 20

expected = 30
  > obj.twenty

expected = 10
  < obj.twenty;

expected = (10
  < obj.twenty)

expected = 10 <
  obj.twenty

notExpected = 10
  < obj.twenty // <-- `<` at start of line misinterpreted as JSX

// syntax highlighting continues to be wonky
let notExpectedFunc = () => { return (true) }
/> // until a JSX closing tag is added
let expectedFunc = () => { return (true) }

Real-world example

In my repo, using prettier for formatting (specifically, a prettier fork that places operators at start of line instead of end) yields the following output, with the problem starting at line 77. Note how placing the cursor on the less-than op highlights the greater-than op on line 84, as if lines 77-84 are just one giant JSX opening tag declaration. Also note that this is a .js file with no mention nor import of React, and the prior examples also had language type explicitly set to JavaScript.

Screenshot from 2022-04-01 11-43-10

My preemptive apologies if this issue is a duplicate (I searched but found none) or if it belongs in a more specific repo.

Dominant language
TypeScript
Stars
471
Forks
149
No merged PRs in 30d

Contributor guide

No contributing guide indexed for this repository

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

More from microsoft/TypeScript-TmLanguage

All issues in microsoft/TypeScript-TmLanguage

Similar issues

More TypeScript issues

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.