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JS
JavaScript error

SyntaxError

The JavaScript exception "property name \_\_proto\_\_ appears more than once in object literal" occurs when an object literal contains multiple occurrences of the proto field, which is used to set the prototype of this new object.

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

The error messages

SyntaxError: Duplicate __proto__ fields are not allowed in object literals (V8-based)
As listed by MDN Web Docs
SyntaxError: property name __proto__ appears more than once in object literal (Firefox)
As listed by MDN Web Docs
SyntaxError: Attempted to redefine __proto__ property. (Safari)
As listed by MDN Web Docs

Details

Category
SyntaxError
Severity
error
Runtime
both

Related errors

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Source: MDN Web Docs. Data fetched 2026-09-25T01:48:37.273Z.