//这里是app.js
onLaunch: function(){
  var that = this;
  that.globalData.onlaunchdesc = "结束";
}
//这里是某page.js
onLoad: async function(){
  var app = getApp();
  function waitForOnlaunchEnd(){
    return new Promise(
      function(resolve, reject){
        while(true){
          if(app.globalData.onlaunchdesc == "结束"){ return resolve() }
        }
      }
    )
  }
  await waitForOnlaunchEnd();
  console.log("onlaunch执行完毕");

  //然后在下面开始写onlaunch执行完毕后，想写的代码
}

没用回掉函数，用了延迟

大神，回调函数有例子吗，发个链接呗，万分感谢，网上百度个回调不管用，看不太懂

你可以用setTimeout来搞；或者使用回调函数来写。