jQuery append方法多参数时，函数参数位置为何影响执行效果？

Great question! Let's dive into why this behavior happens, even though it's not explicitly spelled out in jQuery's official documentation.

The Core Logic Behind jQuery's Insertion Methods #

jQuery's DOM insertion methods like append(), prepend(), after(), and before() share a hidden parameter-handling pattern that explains this quirk:

• When the first parameter is a function, jQuery treats it as a content-generating callback. It runs this function for each matched element, takes its return value, and combines it with any subsequent parameters (treated as static content) to insert into the DOM.
• If a function is passed as any parameter other than the first, jQuery doesn't recognize it as a callback. Instead, it tries to convert the function directly into DOM content—which fails, since raw functions can't be rendered as valid HTML nodes.

Breaking Down Your Test Cases #

Working Example (Function as First Parameter)

$("#append1").click(function(){ 
  $("#list1").append(
    function(){ return "<li>new</li>"; }, 
    "<li>new</li>"
  ); 
})

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<ol id="list1">
<li>old</li>
</ol>
<button id="append1">append1</button>

Here's what happens under the hood:

1. jQuery detects the first argument is a function, so it executes the callback for each element in $("#list1").
2. The callback returns <li>new</li>, which gets combined with the second parameter's static <li>new</li>.
3. Both elements are appended to the list, resulting in two new list items.

Non-Working Example (Function as Second Parameter)

$("#append2").click(function(){ 
  $("#list2").append(
    "<li>new</li>", 
    function(){ return "<li>new</li>"; }
  ); 
})

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<ol id="list2">
<li>old</li>
</ol>
<button id="append2">append2</button>

This fails for a simple reason:

1. jQuery processes the first parameter ("<li>new</li>") as static HTML and appends it to the list.
2. The second parameter is a function, but since it's not the first argument, jQuery doesn't execute it. It attempts to parse the function itself as content, which produces no valid DOM nodes.
3. Only the first static list item gets added—the function's return value never makes it into the DOM.

Why This Isn't Documented #

This edge case is missing from official docs because jQuery's intended usage for these methods is either:

• Passing one or more static content strings/elements, or
• Passing a single function callback to generate dynamic content per element.

Mixing a function with other parameters in any order isn't a commonly promoted use case, hence the lack of explicit documentation. And as you suspected, this behavior isn't unique to append()—it applies to all jQuery methods that share this insertion logic.

内容的提问来源于stack exchange，提问作者Juri Rudi