为什么in运算符对于未定义的属性返回true？

Question

Description

I always wrongly assumed that the in operator was checking for an undefined value for the property. A quick look at the ES5 specification shows that the in operator eventually calls [[GetProperty]] , which would return the value of the property.

Why is it that the when foo.bar is set to undefined the in operator returns true , while when foo.bar is never set it is false , even though they both have a value of undefined ?

Example

var foo = {
    bar: undefined
};

'baz' in foo
// -> false

'bar' in foo
// -> true

Related: typeof foo['bar'] !== 'undefined' vs. 'bar' in foo The related questions was asking about when foo.bar is never set to undefined .

Answer 1

Note: While researching the question I found the answer, since I have already spent the time, I decided to post what I found.

---

The reason why they return a different value is that [[GetProperty]] calls [[GetOwnProperty]] , which does does not return the property value. [[GetOwnProperty]] returns a Property Descriptor which describes things like if the property is enumerable, writable and it's value.

When you set a property to undefined as with bar in the question, a Property Descriptor is created* with a [[Value]] of undefined . When [[GetOwnProperty]] is called for bar the Property Descriptor is returned, while with baz , undefined is returned. This means that [[GetProperty]] return a defined value for bar , which then makes [[HasProperty]] return true , instead of false . Which in turn makes the in operator return true as well.

* this is a lie, but for simplicity sake.

Specification Trace

From Annotated ECMAScript 5.1 .

Note: Numbers below link to Spec reference where possible

For: 'bar' in foo

• 1. Evaluating 'bar' gives a string 'bar'
• 2. GetValue of 'bar' will return the value 'bar'
• 3. Evaluating foo gives a reference foo
• 4. GetValue of foo will return the value of 'foo' , our object
• 5. Type of value of foo is an object, no exception thrown
• 6. toString of value of 'bar' returns the string 'bar , call [[HasProperty]]
     • 1. [[GetProperty]] returns Property Descriptor (not undefined )
     • 1. [[GetOwnProperty]] return Property Descriptor (not undefined )
     • 1. foo does have own property called bar , do not return undefined
     • 2. Create a Property Descriptor with no fields
     • 3. Create X from foo.bar
     • 4. X is a data property, so set [[Value]] to undefined and set [[Writable]]
     • 5. Else branch skipped
     • 6. Set [[Enumerable]]
     • 7. Set [[Configurable]]
     • 8. Return Property Descriptor
     • 2. Property Descriptor is not undefined, return the Property Descriptor.
     • 2. Property Descriptor is not undefined , so don't return false
     • 3. else, return true