为什么此JavaScript数组返回未定义？

Question

Can someone explain when I run this file using node why one value is returning as undefined and the other is not included in the list.

• 0 Cam
• 1 Doug
• 2 Caleb
• 3 David
• 4 Kelli
• 5 Aparna
• 0 Cam
• 1 Doug
• 2 undefined

• 3 David

   var queue = {}; queue[0] = 'Cam'; queue[1] = 'Doug'; queue[2] = 'Caleb'; queue[3] = 'David'; queue[4] = 'Kelli'; queue[5] = 'Aparna'; var sorted_keys = Object.keys(queue).sort(); for (var key in sorted_keys) { console.log(key + " " + queue[key]); } for (var key in sorted_keys) { if (key == 2 || key == 4) { // trying to mock disable accounts and remove delete queue[key]; } } sorted_keys = Object.keys(queue).sort(); for (var key in sorted_keys) { console.log(key + " " + queue[key]); } 

Answer 1

Your main issue is that you're iterating sorted_keys which is an array of keys and you're trying to use the index value from the array to index into the queue object, but that's the array index, not the key into the queue object. You need to get the actual value from the array, not the index from the array because that's where the key into the queue object is.

You also shouldn't be iterating arrays at all with for/in , though that isn't what is actually causing the problem.

If you change your last iteration to this, you will see what you expect:

for (i = 0; i < sorted_keys.length; i++) {
    key = sorted_keys[i];
    console.log(key + " " + queue[key]);
}

Working demo: http://jsfiddle.net/jfriend00/5dPsR/