onreadystatechange、readyState、status是否为JavaScript函数对象的隐式属性？

Great question—let's unpack this step by step to avoid confusion:

First, let's clarify a key distinction:
While all functions in JavaScript are objects, the XMLHttpRequest constructor is a specialized built-in function provided by browsers, and instances like xhr = new XMLHttpRequest() are not generic "function object instances"—they're instances of the XMLHttpRequest type specifically.

Now, to answer your core question: No, these properties are not implicit properties of JavaScript function objects. Here's why:

• These properties belong exclusively to XMLHttpRequest instances (and their prototype chain):
  • readyState and status: These are state-tracking properties defined either directly on the XMLHttpRequest instance or on XMLHttpRequest.prototype. They exist to track the progress and result of the AJAX request, and you won't find them on a generic function object (try running const fn = () => {}; console.log(fn.readyState)—it'll return undefined).
  • onreadystatechange: This is an event handler property (a callback placeholder) defined on XMLHttpRequest.prototype. It's designed to trigger when the request's state changes, and it's unique to XMLHttpRequest-related objects, not all function objects.

To contrast, implicit properties of generic JavaScript function objects are things like:

• prototype: Every function has this property to enable prototype-based inheritance.
• call(), apply(), bind(): Methods inherited from Function.prototype that let you control function execution context.
• name: A property that holds the function's name (if defined).

In short: The properties you're asking about are specific to XMLHttpRequest instances, not inherent to all JavaScript function objects.

内容的提问来源于stack exchange，提问作者donnie darko