JavaScript原生函数参数及jQuery回调参数定义机制问询

Great questions—let’s break this down step by step, starting with the vanilla JavaScript example you provided.

First, let’s recap how addEventListener works:

• It takes 3 parameters (the third is optional):
  1. A string representing the event type (e.g., 'click', 'keydown')
  2. A callback function that runs when the event fires
  3. An optional configuration object or boolean (controls event bubbling/capturing behavior)

What is the event parameter? #

That event (or whatever you name it) is an Event object automatically created and passed to your callback by the browser when the event occurs. It contains all details about the event:

• For a click event, this includes the coordinates of the click, the element that triggered it (event.target), the event type, and methods like preventDefault() (which stops default browser actions, like following a link or submitting a form).

Can we rename it to myGoat? #

Absolutely! Parameter names are just identifiers—you can call it anything you want. The browser only cares that it’s the first parameter of your callback function. Here’s a working example with your silly name:

document.getElementById("example").addEventListener('click', function(myGoat) { 
  myGoat.preventDefault(); // This works perfectly!
})

When/where is this parameter defined? #

You don’t define it manually—the browser handles this. When an event (like a click) is detected, the browser’s event loop triggers the event handling logic: it instantiates an Event (or more specific type, like MouseEvent) object with all the event’s details, then calls your registered callback function, passing this object as the first argument.

Let’s unpack this example next. The $.ajax() method returns a jQuery Deferred object, which manages asynchronous operations like AJAX requests. The .done() method is used to register a callback that runs only when the AJAX request succeeds.

What do response, textStatus, jQueryXMLHttpRequest mean? #

These parameters are automatically passed to your .done() callback by jQuery, in this fixed order:

• response: The data sent back from the server. Its format depends on the dataType you specify (or jQuery will guess based on the server’s response headers—e.g., JSON, HTML, plain text).
• textStatus: A string describing the request’s success state (e.g., "success", "notmodified").
• jQueryXMLHttpRequest: A jQuery-wrapped version of the native XMLHttpRequest object, which includes details like response headers, HTTP status codes, and the raw response data.

Are these parameters tied to .done()? #

Yes, directly. jQuery’s Deferred system handles collecting these values when the AJAX request completes successfully, then passes them in order to any callbacks registered with .done(). Just like with the event parameter, you can rename these too—you just need to keep the order correct:

request = $.ajax({ url: "cre_devis.php", type: "post", data: someData }); 
request.done(function (serverData, statusMsg, xhrObj){ 
  document.getElementById("serverAnswer").innerHTML = serverData; 
});

• For both native event callbacks and jQuery AJAX callbacks: the parameters are created and passed by the underlying environment (browser for events, jQuery for AJAX).
• Parameter names are arbitrary—you can name them whatever makes sense to you, as long as you respect the order of parameters (when multiple are involved).

内容的提问来源于stack exchange，提问作者AniMir