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Strings (string primitives, that is) cannot be modified in JavaScript. You will always receive a new String.

I appreciate the concern for not modifying input data, I agree it's an important subject, but that's why it's also important to know when you can't make mistakes. If you know when you are safe, you also know when you have to pay attention to be safe (i.e., all other cases).

Wrapping the input string in an array is rather useless. Just pass us the string.

The Random tests are generally required. link