TypeError: "x" is (not) "y"

JavaScript 异常“x is (not) y”在出现与期望不符的类型(通常为意外获得的 undefinednull 值)时被抛出。

消息

TypeError: Cannot read properties of undefined (reading 'x') (V8-based)
TypeError: "x" is undefined (Firefox)
TypeError: "undefined" is not an object (Firefox)
TypeError: undefined is not an object (evaluating 'obj.x') (Safari)

TypeError: "x" is not a symbol (V8-based & Firefox)
TypeError: Symbol.keyFor requires that the first argument be a symbol (Safari)

错误类型

什么地方出错了?

出现了与期望不符的类型。这个错误常常由 undefinednull 值引起。

此外,某些方法,例如 Object.create()Symbol.keyFor(),要求必须提供特定类型的参数。

示例

错误情形

// undefined and null cases on which the substring method won't work
const foo = undefined;
foo.substring(1); // TypeError: foo is undefined

const foo = null;
foo.substring(1); // TypeError: foo is null

// Certain methods might require a specific type
const foo = {}
Symbol.keyFor(foo); // TypeError: foo is not a symbol

const foo = 'bar'
Object.create(foo); // TypeError: "foo" is not an object or null

解决方法

要解决值为 undefinednull 的空指针问题,你可以首先测试值是否为 undefinednull

if (foo !== undefined && foo !== null) {
  // Now we know that foo is defined, we are good to go.
}

或者,你如果能确定 foo 的值不会是其它的假值(如:""0),或者排除这些情况不是问题,那你可以简单地测试其是否为真。

if (foo) {
  // Now we know that foo is truthy, it will necessarily not be null/undefined.
}

参见