循环遍历从索引开始的对象

Question

I have an object that is a list of chatrooms, where the keys are chat room names and the value is a reference to the chat room. I want to deliver the rooms to users 50 rooms at a time. But I don't see how this can be done with an object.

Suppose you want to loop through

var obj = {
   "a" : 1,
   "b" : 2.
   "c" : 3
}

But start at "b", ending up with only "b" and "c" being accessed. Can it be done, including using some kind of map or table?

Answer 1

You shouldn't rely on objects keys to be sorted. Although most browsers (if not all) keep them sorted.

So if you want to trust browsers, you can add a counter variable and start doing stuff after the counter reached whatever you want. You can get the length of your object using Object.keys() (returns an array). Mixing both, you can do this:

var object = {
    a: 1,
    b: 2,
    c: 3
};

var keys = Object.keys(object);
var counter;

for (counter = 0; counter < keys.length; counter++) {
    if (counter < 1) continue; // Skip until the second element
    console.log(object[keys[counter]]);
}

If you don't know the position of the key in advance, you can add a flag variable and activate it when you reach the desired key, like this:

var keys = Object.keys(object);
var counter;
var start = false;

for (counter = 0; counter < keys.length; counter++) {
    if (keys[counter] === 'b') start = true;
    if (!start) continue;
    console.log(object[keys[counter]]);
}

Answer 2

Please see the MDN post regarding the for...in loop:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in

Basically, you have no guarantee as to the order in which the keys inside the object will be iterated over. So the idea of skipping a certain "index" simply doesn't apply. I think you need to follow the advice in the comments, and either keep an array of those keys you are interested in, or figure out a way to quickly exclude certain keys as they come up in your loop.

Answer 3

try this way

var obj = {
   'a' : 1,
   'b' : 2.
   'c' : 3
};
var ignoreItems = ['a'];

for (var key in obj) {
  if (ignoreItems.indexOf(key) === -1) {
    // ADD YOUR CODE HERE
  }
}