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

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

"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 with the typeof operator, or to make sure you use the function name, instead of the result of its evaluation.

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

• instanceof
• typeof