类型错误："x" 是（不是）"y"

¥Message

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)

¥Error type

TypeError。

¥What went wrong?

有一种意想不到的类型。对于 undefined 或 null 值，这种情况经常发生。

¥There was an unexpected type. This occurs oftentimes with undefined or null values.

此外，某些方法（例如 Object.create() 或 Symbol.keyFor()）需要必须提供的特定类型。

¥Also, certain methods, such as Object.create() or Symbol.keyFor(), require a specific type, that must be provided.

¥Examples

¥Invalid cases

你不能对 undefined 或 null 变量调用方法。

¥You cannot invoke a method on an undefined or null variable.

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

const foo2 = null;
foo2.substring(1); // TypeError: foo2 is null

某些方法可能需要特定类型。

¥Certain methods might require a specific type.

const foo = {};
Symbol.keyFor(foo); // TypeError: foo is not a symbol

const foo2 = "bar";
Object.create(foo2); // TypeError: "foo2" is not an object or null

¥Fixing the issue

要将空指针修复为 undefined 或 null 值，可以先测试该值是 undefined 还是 null。

¥To fix null pointer to undefined or null values, you can test if the value is undefined or null first.

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

或者，如果你确信 foo 不会是像 "" 或 0 这样的另一个 falsy 值，或者如果过滤掉这些情况不是问题，你可以简单地测试其真实性。

¥Or, if you are confident that foo will not be another falsy value like "" or 0, or if filtering those cases out is not an issue, you can simply test for its truthiness.

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

¥See also

• undefined
• null