Web editor: "forEach is not a function" — array loses prototype methods in test runner

Programming JavaScript
,
1

Describe the bug
When running tests in the web editor, array methods like forEach are unavailable inside the solution function. The test runner reports TypeError: birdsPerDay.forEach is not a function, even though the passed argument appears to be an array in console.log.

To Reproduce

  1. Open the “Bird Watcher” exercise in the web editor (JavaScript track).
  2. Submit a solution using forEach:

export function totalBirdCount(birdsPerDay) {

let total = 0;

birdsPerDay.forEach(element => { total += element; });

return total;

}

  1. Click “Run Tests”.
  2. See error: TypeError: birdsPerDay.forEach is not a function

Console output also shows:

[worker] could not be made Qh: /javascript-browser-test-runner-worker.mjs did not yield a valid worker

Expected behavior
forEach should work normally on the array passed by the test runner.

Workaround
Using a for loop with index access works, since it doesn’t rely on array prototype methods.

Environment

  • Browser: Chrome (with DevTools open)
  • Track: JavaScript
  • Exercise: Bird Watcher
2

The instructions say,

To practice, use a for loop to solve each of the tasks below.

and

Implement a function totalBirdCount that accepts an array-like object that contains the bird count per day.

This is by design and intentional. You should be using for.

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