FES error masks actual undefined variable ReferenceError

[Aayushdev18]

Most appropriate sub-area of p5.js?

• Accessibility
• Color
• Core/Environment/Rendering
• Data
• DOM
• Events
• Image
• IO
• Math
• Typography
• Utilities
• p5.strands
• WebGL
• DevOps, Build process, Unit testing
• Internationalization (i18n)
• Friendly Errors
• Other (specify if possible)

p5.js version

2.1.2

Web browser and version

Chrome

Operating system

macOS

Steps to reproduce this

Steps:

1. Open the example sketch: https://editor.p5js.org/davepagurek/sketches/aVaqe0sNn
2. Run the sketch (or create a new sketch with the code below)
3. Open the browser console
4. Observe that two errors appear instead of one

Problem:
When a variable is undefined (e.g., due to a typo or forgetting to define it), p5.js shows two errors:

1. An error from FES trying to make an error
2. The actual ReferenceError (e.g., "s is not defined")

The first FES error masks the real error message, making debugging harder.

Expected behavior:
Users should see a single, clear error message indicating the undefined variable.

Actual behavior:
Two errors appear — the FES error and the actual ReferenceError.

Additional context:
This is not p5.strands-specific; it also happens in regular p5.js.

Snippet:

warp = baseStrokeShader().modify(() => {
  const frame = uniformFloat(() => frameCount)

  getObjectInputs((inputs) => {
    inputs.position.xy += 20 * noise(
      inputs.position.x * s,
      inputs.position.y * s,
      frame * 0.01
    )
  })

  return inputs
})


Note:
The variable s is intentionally undefined in this example.

[Aayushdev18]

@davepagurek Thanks for identifying this issue! I'm ready to work on fixing this.
I'll start by investigating the FES error handling in fesErrorMonitor() to understand why it's generating an error that masks the actual ReferenceError.
Could you please assign this issue to me? I'll investigate the code and create a PR once I have a solution.
Thanks.