如何确定一个js 是否加载完全或者页面中的所有js加载完全，并且使脚本的执行顺序按照设定的顺序执行？

//封装loadscript方法
export function loadScript(url, callback) {
    var script = document.createElement("script");
    script.type = "text/javascript"
    if (script.readyState) {
        script.onreadystatechange = function () {
            if (script.readyState == "loaded" || script.readyState == "complete") {
                script.onreadystatechange = null
                callback()
            }
        }
    } else {
        script.onload = function () {
            callback()
        }
    }
    script.src = url
    document.getElementsByName("head")[0].appendChild(script)
}
//嵌套使用
loadScript("file1.js", function () {
    loadScript("file2.js", function () {
        loadScript("file3.js", function () {
            alert("All files are loaded!")
        })
    })
})