var elem = document.createElement('link');
elem.rel = ' stylesheet'
elem.href= 'style.css';//Link of the css file
document.head.appendChild(elem);

下面是jQuery的元素创建方法(我的偏好)和回调onLoad的方法:

var css = $("<link>", {
  "rel" : "stylesheet",
  "type" :  "text/css",
  "href" : "style.css"
})[0];

css.onload = function(){
  console.log("CSS IN IFRAME LOADED");
};

document
  .getElementsByTagName("head")[0]
  .appendChild(css);

var fileref = document.createElement("link")
fileref.setAttribute("rel", "stylesheet")
fileref.setAttribute("type", "text/css")
fileref.setAttribute("th:href", "@{/filepath}")
fileref.setAttribute("href", "/filepath")

我用的是百里香叶，效果很好。谢谢

这个函数使用记忆。并且可以多次调用，而不会出现两次加载和运行相同样式表的冲突。而且，它不会在样式表实际加载之前解析。

const loadStyle = function () {
    let cache = {};
    return function (src) {
        return cache[src] || (cache[src] = new Promise((resolve, reject) => {
            let s = document.createElement('link');
            s.rel = 'stylesheet';
            s.href = src;
            s.onload = resolve;
            s.onerror = reject;
            document.head.append(s);
        }));
    }
}();

请注意函数表达式后面的括号()。

样式表的并行加载:

Promise.all([
    loadStyle('/style1.css'),
    loadStyle('/style2.css'),
    // ...
]).then(() => {
    // do something
})

您可以对动态加载脚本使用相同的方法。

在现代浏览器中，您可以使用这样的promise。创建一个带有promise的loader函数:

function LoadCSS( cssURL ) {

    // 'cssURL' is the stylesheet's URL, i.e. /css/styles.css

    return new Promise( function( resolve, reject ) {

        var link = document.createElement( 'link' );

        link.rel  = 'stylesheet';

        link.href = cssURL;

        document.head.appendChild( link );

        link.onload = function() { 

            resolve(); 

            console.log( 'CSS has loaded!' ); 
        };
    } );
}

显然，你想在CSS加载后完成一些事情。你可以像这样调用CSS加载后需要运行的函数:

LoadCSS( 'css/styles.css' ).then( function() {

    console.log( 'Another function is triggered after CSS had been loaded.' );

    return DoAfterCSSHasLoaded();
} );

如果你想深入了解它是如何工作的，有用的链接:

官方承诺文件

有关承诺的有用指南

一个很棒的关于承诺的介绍视频

下面是用于加载JS和/或CSS的完整代码

function loadScript(directory, files){
  var head = document.getElementsByTagName("head")[0]
  var done = false
  var extension = '.js'
  for (var file of files){ 
    var path = directory + file + extension 
    var script = document.createElement("script")
    script.src = path        
    script.type = "text/javascript"
    script.onload = script.onreadystatechange = function() {
        if ( !done && (!this.readyState ||
            this.readyState == "loaded" || this.readyState == "complete") ) {
            done = true
            script.onload = script.onreadystatechange = null   // cleans up a little memory:
            head.removeChild(script)  // to avoid douple loading
        }
  };
  head.appendChild(script) 
  done = false
 }
}

function loadStyle(directory, files){
  var head = document.getElementsByTagName("head")[0]
  var extension = '.css'
  for (var file of files){ 
   var path = directory + file + extension 
   var link = document.createElement("link")
   link.href = path        
   link.type = "text/css"
   link.rel = "stylesheet" 
   head.appendChild(link) 
 }
}

(() => loadScript('libraries/', ['listen','functions', 'speak', 'commands', 'wsBrowser', 'main'])) ();
(() => loadScript('scripts/', ['index'])) ();

(() => loadStyle('styles/', ['index'])) ();