javascript - 为什么在条件中不检查流密封对象类型属性

Question

条件语句中未检查密封对象属性的原因是什么？

例如:

// Create a sealed object type
const o = {
  Foo: 'Foo',
};

// Access a non-existent property
// Reports an Error
const fooz = o.Fooz;

// Conditional with non-existent property
// Does not report an error
if (o.Fooz === undefined) {}

澄清一下 - 这是 Flow sealed object

Answer 1

我将此问题发布为 Flow issue 。我收到回复:

Non-existence is not an error within a conditional expression. See the comment from the source.

源评论如下:

Conditional expressions are checked like expressions, except that property accesses are provisionally allowed even when such properties do not exist. This accommodates the common JavaScript idiom of testing for the existence of a property before using that property.