TypeError: invalid 'instanceof' operand 'x'
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 aprototype
property.)
Examples
instanceof vs. typeof
js
"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.
js
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