为什么这个 Javascript 返回 undefined？

Question

var linksList = [
   "http://a.com",
   "http://b.com",
   "http://c.com",
]

for (var i=0; i<linksList.length; i++) {
    setTimeout(function() {
        console.log(linksList[i]); 
    }, 3000);
}  

I'm pasting this into Chrome inspector and it returns a number for whatever reason (it seems to be an entirely random number?)... then it waits... and then returns 3 'undefined' console errors.

Answer 1

Use closure:

for (var i=0; i<linksList.length; i++) {
    (function(i){
       setTimeout(function() {
          console.log(linksList[i]); 
       }, 3000);
    })(i);
}

Your problem is ,when setTimeout 's callback is called, i = linksList.length already , so you are outputting:

 console.log(linksList[ linksList.length ]); 

3 times, where linksList[ linksList.length ] is undefined .