TypeError: invalid 'instanceof' operand 'x'

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Message

TypeError: invalid 'instanceof' operand "x" (Firefox) 
TypeError: "x" is not a function (Firefox) 
TypeError: Right-hand side of 'instanceof' is not an object (Chrome) 
TypeError: Right-hand side of 'instanceof' is not callable (Chrome)

Error type

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.

Examples

"test" instanceof ""; // TypeError: invalid 'instanceof' operand ""
42 instanceof 0;      // TypeError: invalid 'instanceof' operand 0

function Foo() {}
var f = Foo();        // Foo() is called and returns undefined
var 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() {}
var f = Foo;               // Do not call Foo.
var x = new Foo();

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

See also