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

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

Examples:
TypeError: "x" is undefined
TypeError: "x" is null
TypeError: "undefined" is not an object
TypeError: "x" is not an object or null
TypeError: "x" is not a symbol

TypeError.

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

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

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

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


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

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

要解决空指针以及 undefined 或 null 值的问题， 你可以使用 typeof 操作符， 例如：

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

• undefined
• null