Length of a JavaScript object

If I have a JavaScript object, say

var myObject = new Object();
myObject["firstname"] = "Gareth";
myObject["lastname"] = "Simpson";
myObject["age"] = 21;

is there a built-in or accepted best practice way to get the length of this object?

---

The most robust answer (ie that captures the intent of what you're trying to do while causing the fewest bugs) would be:

Object.size = function(obj) {
    var size = 0, key;
    for (key in obj) {
        if (obj.hasOwnProperty(key)) size++;
    }
    return size;
};

// Get the size of an object
var size = Object.size(myArray);

There's a sort of convention in JavaScript that you don't add things to Object.prototype, because it can break enumerations in various libraries. Adding methods to Object is usually safe, though.

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Here's an update as of 2016 and widespread deployment of ES5 and beyond. For IE9+ and all other modern ES5+ capable browsers, you can use Object.keys() so the above code just becomes:

var size = Object.keys(myObj).length;

This doesn't have to modify any existing prototype since Object.keys() is now built in.

---

如果你知道你不必担心hasOwnProperty检查，你可以很简单地做到这一点：

Object.keys(myArray).length

---

Updated : If you're using Underscore.js (recommended, it's lightweight!), then you can just do

_.size({one : 1, two : 2, three : 3});
=> 3

If not , and you don't want to mess around with Object properties for whatever reason, and are already using jQuery, a plugin is equally accessible:

$.assocArraySize = function(obj) {
    // http://stackoverflow.com/a/6700/11236
    var size = 0, key;
    for (key in obj) {
        if (obj.hasOwnProperty(key)) size++;
    }
    return size;
};

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