Documentación offline JavaScript main
/ ó ⌘K

TypeError: invalid 'instanceof' operand 'x'

main Documentación oficial Licencia CC-BY-SA-2.5Descargado el 2026-08-02

En esta página

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.

Message#

TypeError: Right-hand side of 'instanceof' is not an object (V8-based)
TypeError: invalid 'instanceof' operand "x" (Firefox)
TypeError: Right hand side of instanceof is not an object (Safari)

TypeError: Right-hand side of 'instanceof' is not callable (V8-based)
TypeError: x is not a function (Firefox)
TypeError: x is not a function. (evaluating 'x instanceof y') (Safari)

TypeError: Function has non-object prototype 'undefined' in instanceof check (V8-based)
TypeError: 'prototype' property of x is not an object (Firefox)
TypeError: instanceof called on an object with an invalid prototype property. (Safari)

Error type#

{{jsxref("TypeError")}}

What went wrong?#

The instanceof operator expects the right-hand-side operands to be a constructor object, i.e., an object which has a prototype property and is callable. It can also be an object with a Symbol.hasInstance method. This error can occur if:

  • The right-hand side operand is not an object.
  • The right-hand side operand is not a callable and it has no Symbol.hasInstance method.
  • The right-hand side operand is a callable, but its prototype property is not an object. (For example, arrow functions do not have a prototype property.)

Examples#

instanceof vs. typeof#

```js example-bad "test" instanceof ""; // TypeError: invalid 'instanceof' operand "" 42 instanceof 0; // TypeError: invalid 'instanceof' operand 0

function Foo() {} const f = Foo(); // Foo() is called and returns undefined const x = new Foo();

x instanceof f; // TypeError: invalid 'instanceof' operand f x instanceof x; // TypeError: x is not a function

To fix these errors, you will either need to replace
the [`instanceof` operator](/en-US/docs/Web/JavaScript/Reference/Operators/instanceof)
with the [`typeof` operator](/en-US/docs/Web/JavaScript/Reference/Operators/typeof),
or to make sure you use the function name, instead of the result of its evaluation.

```js example-good
typeof "test" === "string"; // true
typeof 42 === "number"; // true

function Foo() {}
const f = Foo; // Do not call Foo.
const x = new Foo();

x instanceof f; // true
x instanceof Foo; // true

See also#