function loadScript(url, callback) {
    var script = document.createElement("script")
    script.type = "text/javascript";
    if (script.readyState) { //IE 
      script.onreadystatechange = function () {
        if (script.readyState == "loaded" || script.readyState == "complete") {
          script.onreadystatechange = null;
          callback();
        }
      };
    } else {
      script.onload = function () {
        callback();
      };
    }
    script.src = url;
    document.getElementsByTagName("head")[0].appendChild(script);
  };

  loadScript("file3.js", function () {
    loadScript("file1.js", function () {
      loadScript("file2.js", function () {
        console.log("All files are loaded!");
      });
    });
  });

jq：

$.getScript();
         $.getScript("test.js",function(){
                setTimeout(function(){
                    alert(2)
                },2000)
            })

 

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