当我在DOM对象上查看getter属性时，控制台中发生了什么？

Question

When running the following code in the console:

console.dir(document);

In Chrome, I see, among other things:

This seems to imply that the domain property is directly on the document object. However, it isn't.

 console.log(document.hasOwnProperty('domain')); 

In Chrome 72, going up the prototype chain, it appears to be on Document.prototype :

 console.log(Document.prototype.hasOwnProperty('domain')); console.log(Object.getOwnPropertyDescriptor(Document.prototype, 'domain')); 

(In FF 56 and perhaps some other browsers, it appears to be on HTMLDocument.prototype instead)

As you can see from the snippet, the property is actually composed of a getter and a setter. But, I was under the impression that getters are shown in the console as (...) , like in this image , which you had to click on the (...) to invoke the getter.

If I create a similar object, one whose prototype contains a getter/setter property, and I log the object, the getter does not get invoked while examining it:

 // look at results in Chrome's browser console, not snippet console class theProto { get foo() { return 'fooVal'; } set foo(arg) { // do something } } class theClass extends theProto { } const instance = new theClass(); console.dir(instance); 

The same sort of behavior can be seen for many properties on document . For example, all of the other properties you can see in the first screenshot also appear to be getters/setters on one of the prototype objects, and none of them are on document itself:

 console.log( ['dir', 'doctype', 'documentElement', 'documentURI', 'embeds', 'fgColor', 'firstChild', 'firstElementChild'] .some(prop => document.hasOwnProperty(prop)) ); 

You can also see this on window properties, as well as elements. This happens in FF as well.

 const input = document.createElement('input'); // console.dir(input); // but the own property list is empty! console.log(Object.getOwnPropertyNames(input)); 

 <img src="https://i.stack.imgur.com/R5u3S.png"> 

Is it possible to create an object with the same logging behavior as these, where console.dir ing an object will also invoke any getters in the prototype chain immediately, instead of displaying (...) ? How would I modify my theClass snippet? Or, do certain predefined objects (like DOM objects) just have an exception to the normal logging behavior?

I know how to invoke the getters programmatically, I'm just curious about the seeming inconsistency.

Answer 1

Is it possible to create an object with the same logging behavior as these, where console.diring an object will also invoke any getters in the prototype chain immediately, instead of displaying (...)?

Thecnically, yes, despite it will not exactly evaluate the getters (I'm not sure whether the values you see are evaluated when you console.dir them). You need to evaluate the property. However, using hasOwnProperty will return true.

 // look at results in Chrome's browser console, not snippet console class theProto { get foo() { return 'fooVal'; } set foo(arg) { // do something } } class theClass extends theProto { foo = (() => theProto.prototype.foo)(); // or, really, just (() => this.foo)(); } const instance = new theClass(); console.dir(instance); 

Working fiddle: https://jsfiddle.net/vL6smo51/1/

Answer 2

There's a few things to be aware of.

1. First is that the console.* statements (other than .error) are async, you are passing the document object by reference that may in fact change between passing it to console.FN and the actual output to the log window.
2. Second, is that the browser has access to more than you might via the JS API directly in terms of interrogation.
3. Third is that document itself may have an inheritance chain that includes things like domain which is not a part of the document instance itself.
4. Fourth is that not all properties are iterable by design, but may still show up in the log (see #2 above)

Answer 3

document.hasOwnProperty() statement only returns true if the object has the specified property as its own property (as opposed to inheriting it). In this cause domain property is inherited from HTMLDocument object. HTMLDocument is the prototype of document. So it returns false.