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JavaScript error

TypeError

The JavaScript exception "invalid 'instanceof' operand" occurs when the right-hand side operands of the instanceof operator isn't used with a constructor object, i.e., an object which has a prototype property and is callable.

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

The error messages

TypeError: Right-hand side of 'instanceof' is not an object (V8-based)
As listed by MDN Web Docs
TypeError: invalid 'instanceof' operand "x" (Firefox)
As listed by MDN Web Docs
TypeError: Right hand side of instanceof is not an object (Safari)
As listed by MDN Web Docs
TypeError: Right-hand side of 'instanceof' is not callable (V8-based)
As listed by MDN Web Docs
TypeError: x is not a function (Firefox)
As listed by MDN Web Docs
TypeError: x is not a function. (evaluating 'x instanceof y') (Safari)
As listed by MDN Web Docs
TypeError: Function has non-object prototype 'undefined' in instanceof check (V8-based)
As listed by MDN Web Docs
TypeError: 'prototype' property of x is not an object (Firefox)
As listed by MDN Web Docs
TypeError: instanceof called on an object with an invalid prototype property. (Safari)
As listed by MDN Web Docs

Details

Category
TypeError
Severity
error
Runtime
both

Related errors

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