[I have trouble using timelinemax add() function to create a callback function]

Thanks @mikel, I have tested my thoughts, the tl.time() could compare labels'time well.

[I have trouble using timelinemax add() function to create a callback function]

I think I have find one way. Using the tl.time() to compare the label's time to judge reach which label.

btn.addEventListener('click', function(){
  var tl=new TimelineMax();
  var obj=$("#entrance");
  for(var i=0;i<4;i++){
    var label="mylabel_"+i;
    tl.addLabel(label);
    if(i%2==1){
      label="mylabel_"+i;
      tl.to(obj,5,{x:100+i*100, onUpdate:
                function(){
      var panel=document.getElementById("test");
      var t=tl.time().toFixed(2);
      for (var i = 0; i <labels.length; i++) {
        if(t>labels[i].time&&t<labels[i+1].time){
          panel.innerHTML=labels[i].name;
          break;
        }
       }; 
    }});
    }else{
      tl.to(obj,5,{y:100+i*100, onUpdate:
                function(){
      var panel=document.getElementById("test");
      var t=tl.time().toFixed(2);
      for (var i = 0; i <labels.length; i++) {
        if(t>labels[i].time&&t<labels[i+1].time){
          panel.innerHTML=labels[i].name;
          break;
        }
       };       
    }});
    }
  }
  tl.addLabel("mylabel_final");
  var labels = tl.getLabelsArray();
    for (var i = 0; i < labels.length; i++) {
    console.log("label name: " + labels[i].name + ", time: " + labels[i].time);
  };
});

But i'm not sure when using timescale() to change the play speed ,the tl.time() could compare labels'time well.

[I have trouble using timelinemax add() function to create a callback function]

Thanks mikel.The timeline has four animations,each of them has its own label.What i want to achieve are their labels.When each animation start , i want to use its label to change the panel's innerHTML.Just like your example,the value only show the last label.How to show the label "mylabel_0","mylabel_1","mylabel_2"when the timeline play?

[I have trouble using timelinemax add() function to create a callback function]

As the example,I want to use add() to create a callback function at each timeline's beginning,and use this to change the describe.But when it run,the callback function can't get the right value and the i became undefinded.  

See the Pen zgrwJa by QINGYE (@QINGYE) on CodePen

[How can i let multiple timelinelite objects work together]

Thank you,I find the problem.Just as you say,I use the same timline,so the animation play one by one.I change my object array and solve the problem.

for (var i = 0; i < initnum; i++) { //init
      car[i] = {
        ...
        tl: new TimelineLite()
      };
}

 

[How can i let multiple timelinelite objects work together]

Thanks Jack,I have studied the tutorial which you given , but it can not solve my problem.Just like the following codes：

var t = new TimelineLite();
var car = new Array();
for (var i = 0; i < initnum; i++) { //init
      car[i] = {
        id:xxx,
        name:xxx,        
        tl: t
     };
}

I give each car object a timeline and I want to play these timelines together.However I can only play these timelines one by one.

for(var i=0;i<initnum;i++)
{
  ...
for (var j = 0; j <xxx; j++) 
{
     ....
     car[i].tl.to(obj,...  );
     car[i].tl.to(obj,... );
 }
car[i].tl.play();
}

So how can I to play these timelines together.

[How can i let multiple timelinelite objects work together]

I need to show multiply cars' real-time track,so i try to create multiply timelinelite objects to fit cars.But when play these timelinelite objects,i find that they play one by one otherwise play together.How can i solve this problem.