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`typescript` preset: incompatible with `+=` operator inside arrow functions, nested in ternary #11038

buschtoens opened this issue Jan 21, 2020 · 2 comments


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@buschtoens buschtoens commented Jan 21, 2020

Bug Report

  • I would like to work on a fix!
    I have close to zero experience with Babel's internals though. I would not be offended, if you think you can fix it yourself quicklier.

Current Behavior
The typescript preset is incompatible with d3-array/cumsum.js. The issue seems to only occur for arrow functions using the += operator, nested inside a ternary. When typescript is disabled, the code compiled without any issues.

Input Code

  • Original broken d3-array/cumsum.js: REPL
  • Minimal reproduction: REPL
0 ? v => (sum += v) : v => 0;

/repl.ts: Only '=' operator can be used for specifying default value. (1:13)

1 | 0 ? v => (sum += v) : v => 0;

Expected behavior/code
Babel should compile the above code with typescript enabled.


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@babel-bot babel-bot commented Jan 21, 2020

Hey @buschtoens! We really appreciate you taking the time to report an issue. The collaborators on this project attempt to help as many people as possible, but we're a limited number of volunteers, so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack community that typically always has someone willing to help. You can sign-up here for an invite."


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@jridgewell jridgewell commented Jan 21, 2020

To help you narrow it down, it's an issue with the parser and likely something in packages/babel-parser/src/plugins/typescript/index.js.

@jridgewell jridgewell added the claimed label Jan 21, 2020
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