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: 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
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
instanceof vs typeof
"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