[JS/TS] Negating a -0.0 literal emits --0 and fails with SyntaxError

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
77/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
javascript, typescript
Domain
compilers

Research direction

Start by locating the unary-minus parenthesization check described in the issue, which currently tests whether the value is less than 0.0, and trace how it handles the -0.0 literal. Use the repro to confirm the generated JavaScript or TypeScript no longer contains invalid --0, then verify the output is valid and evaluates to 0.000000.

Written by the indexing model from the issue text.

Description

Description

Negating a -0.0 float literal (directly or after inlining) emits --0 in JavaScript and TypeScript output, which fails at parse time as a prefix decrement on a literal because the unary-minus parenthesization check only tests value < 0.0 (which is false for -0.0).

Repro code
let x = -(-0.0)
printfn "%f" x
Expected and actual results

Expected result:

Compiles to valid JavaScript/TypeScript (e.g. const x = -(-0); or 0), and outputs 0.000000 like .NET F#.

Actual result:

Fable compiles the binding to const x = --0;, which Node.js rejects at parse time with:

SyntaxError: Invalid left-hand side expression in prefix operation
Related information
  • Fable version: 5.27.0
  • Operating system: macOS / Linux

Credit: https://ucsbplse.github.io/

Dominant language
F#
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