Regression where JS objects are mutated with polyfill code implementation

[InterLinked1]

Noticed some strange behavior in https://github.com/InterLinked1/wssmail, since commit a71389a

webmail.js:1913 Uncaught TypeError: Cannot read properties of undefined (reading 'indexOf')
    at displayFolderName (webmail.js:1913:26)
    at addToFolderMenu (webmail.js:606:17)
    at drawFolderMenu (webmail.js:1974:3)
    at handleMessage (webmail.js:2244:4)
    at ws.onmessage (webmail.js:451:3)

Attempting to print the folders object/array in the console succeeds but also has "undefined" after it.

When logging the individual stray objects that appear, the code in the toReversed and 2x the toSpliced polyfills appears as the object.