确定Javascript对象有多少个字段

Question

I have a Javascript object that I'm trying to use as a "hashmap". The keys are always strings, so I don't think I need anything as sophisticated as what's described in this SO question . (I also don't expect the number of keys to go above about 10 so I'm not particularly concerned with lookups being O(n) vs. O(log n) etc.)

The only functionality I want that built-in Javascript objects don't seem to have, is a quick way to figure out the number of key/value pairs in the object, like what Java's Map.size returns. Of course, you could just do something like:

function getObjectSize(myObject) {
  var count=0
  for (var key in myObject)
    count++
  return count
}

but that seems kind of hacky and roundabout. Is there a "right way" to get the number of fields in the object?

Answer 1

There is an easier way spec'd in ECMAScript 5.

Object.keys(..) returns an array of all keys defined on the object. Length can be called on that. Try in Chrome:

Object.keys({a: 1, b: 2}).length; // 2

Note that all objects are basically key/value pairs in JavaScript, and they are also very extensible. You could extend the Object.prototype with a size method and get the count there. However, a much better solution is to create a HashMap type interface or use one of the many existing implementations out there, and define size on it. Here's one tiny implementation:

function HashMap() {}

HashMap.prototype.put = function(key, value) {
    this[key] = value;
};

HashMap.prototype.get = function(key) {
    if(typeof this[key] == 'undefined') {
        throw new ReferenceError("key is undefined");
    }
    return this[key];
};

HashMap.prototype.size = function() {
    var count = 0;

    for(var prop in this) {
        // hasOwnProperty check is important because 
        // we don't want to count properties on the prototype chain
        // such as "get", "put", "size", or others.
        if(this.hasOwnProperty(prop) {
            count++;
        }
    }

    return count;
};

Use as ( example ):

var map = new HashMap();
map.put(someKey, someValue);
map.size();

Answer 2

A correction: you need to check myObject.hasOwnProperty(key) in each iteration, because there're can be inherited attributes. For example, if you do this before loop Object.prototype.test = 'test' , test will aslo be counted.

And talking about your question: you can just define a helper function, if speed doesn't matter. After all, we define helpers for trim function and other simple things. A lot of javascript is "kind of hacky and roundabout" :)

update
Failure example, as requested.

Object.prototype.test = 'test';
var x = {};
x['a'] = 1;
x['b'] = 2;

The count returned will be 3.

Answer 3

you could also just do myObject.length (in arrays)

nevermind, see this: JavaScript object size

Answer 4

That's all you can do. Clearly, JavaScript objects are not designed for this. And this will only give you the number of Enumerable properties. Try getObjectSize(Math) .