worker.onmessage 何时被触发？

<script>
var workers=[];
var btn = document.getElementById("btn");
btn.onclick = function(){

for(var i=0;i<5;i++){

var worker =new Worker("getticket.js");
 worker.onmessage = getmessage;
workers.push(worker);}}

function getmessage(event){

    var w =event.target;
    var index = workers.indexOf(w);
    var div = document.getElementById("div"+index);
    if(event.data == 1){
        div.className="getticket";}
    else{
        div.className ="noticket"}}

</script>

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当你postmessage的时候触发。。

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各位大神，我也想知道正确答案