SyntaxError: for-in loop head declarations may not have initializers

This is a reference entry from MDN Web Docs. A full explanation with fixes has not been written yet.

The error messages

SyntaxError: for-in loop variable declaration may not have an initializer. (V8-based)

As listed by MDN Web Docs

SyntaxError: for-in loop head declarations may not have initializers (Firefox)

As listed by MDN Web Docs

SyntaxError: a lexical declaration in the head of a for-in loop can't have an initializer (Firefox)

As listed by MDN Web Docs

SyntaxError: Cannot assign to the loop variable inside a for-in loop header. (Safari)

As listed by MDN Web Docs

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Source: MDN Web Docs. Data fetched 2026-09-25T01:48:37.274Z.