The function isBig will only return a specific value when its parameter is bigger than
5000. In any other case, it will implicitly return undefined.
This behaviour may not be what you had intended. In any case, you can add a
returnundefined to the other execution path to make the return value
explicit.
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return 'Are you sure you want to leave this page?';
This checks looks for references to variables that have not been declared. This is most likey a typographical error or a variable has been renamed.
To learn more about declaring variables in Javascript, see the MDN.