TypeError: Right-hand side of 'instanceof' is not an object (V8-based)
TypeError: Right-hand side of 'instanceof' is not callable (V8-based)
TypeError: invalid 'instanceof' operand "x" (Firefox)
TypeError: ({}) is not a function (Firefox)
TypeError: Right hand side of instanceof is not an object (Safari)
TypeError: {} is not a function. (evaluating '"x" instanceof {}') (Safari)

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.

"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

• instanceof
• typeof