Is the illegal shadowing error a SyntaxError or a ReferenceError in JavaScript?
SyntaxError. The engine catches the duplicate declaration at parse time, before any code runs. The message is 'Identifier x has already been declared.'
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More FAQs in Illegal Shadowing in JavaScript Explained
You cannot shadow a let, const, or class with a var in the same scope. var is function-scoped and leaks out of blocks, so it would conflict with the outer let/const/class. The engine throws SyntaxError: Identifier has already been declared.
Yes. Shadowing a var with a let is legal because let is block-scoped and does not leak out. There is no conflict. The reverse (shadowing a let with a var) is illegal.
Because var ignores block boundaries and would leak into the scope where the let is declared, creating two declarations of the same name in the same scope. The engine catches this at parse time.
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