• @Gordon—Oh! Mea culpa. I thought I had ruled out internal minification, but I had neglected to defer execution in my experiments, which rather invalidates them :-}. Sorry!

    Here are the ones I can think of:

    Misinterpreting valid and useful code:
    - - / --
    + +/++

    Wildly misinterpreting technically valid but extremely unlikely code:
    / / / // ⇐ Humorously, the bizarre sequence 1 / /a/ does indeed throw an amazing fit, and can produce arbitrarily inappropriate error messages—add a multiline template literal later in the function for total hilarity!

    Incorrectly allowing syntactically invalid code in an especially confusing way:
    /…/ i / /…/i ⇐ The IDE flags /a/ i as a syntax error, but the runtime sees /a/i.

    The obscure example do x(); while(…); is already handled correctly, and sequences with - -- and + ++ display incorrectly in error messages but apparently execute correctly—are the increment and decrement operators internally pretokenised, or something?

    I can't think of any other examples because alphanumeric tokens already preserve whitespace and JS has few cases where an operator symbol is at the left edge of an expression. Maybe ** and yield*, should they ever be implemented, might surprise people carelessly converting C code?

    HTH.

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