类型错误：'instanceof' 操作数 'x' 无效

¥Message

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)

¥Error type

¥What went wrong?

instanceof 运算符 期望右侧操作数是构造函数对象，即具有 prototype 属性并且可调用的对象。它也可以是具有 Symbol.hasInstance 方法的对象。

¥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.

¥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

要修复这些错误，你需要将 instanceof 运算符 替换为 typeof 运算符，或者确保使用函数名称，而不是其计算结果。

¥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