How do you determine equality for two JavaScript objects?

Question

A strict equality operator will tell you if two object types are equal, however, is there any way to tell if two objects are equal, much like the hash code value in Java?

This is similar to this question, but requires a more academic answer:

http://stackoverflow.com/questions/194846/is-there-any-kind-of-hashcode-function-in-javascript

The scenario above demonstrates why it would be necessary to have one, and I'm wondering if there is any equivalent solution.

Answer 1

Depends on what you mean by equality. And therefore it is up to you, as the developer of the classes, to define their equality.

There's one case used sometimes, where two instances are considered 'equal' if they point to the same location in memory, but that is not always what you want. For instance, if I have a Person class, I might want to consider two Person objects 'equal' if they have the same Last Name, First Name, and Social Security Number (even if they point to different locations in memory).

On the other hand, we can't simply say that two objects are equal if the value of each of their members is the same, since, sometimes, you don't want that. In other words, for each class, it's up to the class developer to define what members make up the objects 'identity' and develop a proper equality operator (be it via overloading the == operator or an Equals method).

Saying that two objects are equal if they have the same hash is one way out. However you then have to wonder how the hash is calculated for each instance. Going back to the Person example above, we could use this system if the hash was calculated by looking at the values of the First Name, Last Name, and Social Security Number fields. On top of that, we are then relying on the quality of the hashing method (that's a huge topic on its own, but suffice it to say that not all hashes are created equal, and bad hashing methods can lead to more collisions, which in this case would return false matches).

Answer 2

Are you trying to test if two objects are the equal? ie: their properties are equal?

If this is the case, you'll probably have noticed this situation:

var a = { foo : "bar" };
var b = { foo : "bar" };
alert (a == b ? "Equal" : "Not equal");
// "Not equal"

you might have to do something like this:

function objectEquals(obj1, obj2) {
    for (var i in obj1) {
        if (obj1.hasOwnProperty(i)) {
            if (!obj2.hasOwnProperty(i)) return false;
            if (obj1[i] != obj2[i]) return false;
        }
    }
    for (var i in obj2) {
        if (obj2.hasOwnProperty(i)) {
            if (!obj1.hasOwnProperty(i)) return false;
            if (obj1[i] != obj2[i]) return false;
        }
    }
    return true;
}

Obviously that function could do with quite a bit of optimisation, and the ability to do deep checking (to handle nested objects: var a = { foo : { fu : "bar" } }) but you get the idea.

As FOR pointed out, you might have to adapt this for your own purposes, eg: different classes may have different definitions of "equal". If you're just working with plain objects, the above may suffice, otherwise a custom MyClass.equals() function may be the way to go.

Answer 3

If you are using a JSON library, you can encode each object as JSON, then compare the resulting strings for equality.

var obj1={test:"value"};
var obj2={test:"value2"};

alert(JSON.encode(obj1)===JSON.encode(obj2));

Answer 4

The short answer

The simple answer is no there is no generic means to determine that an object is equal to another in the sense you mean. The exception is when you are strictly thinking of an object being typeless.

The long answer

The concept is that of an Equals method that compares two different instances of an object to indicate whether at a value level they are equal. However it is upto the specific type to define how an Equals method should be implemented. An iterative comparision of attributes that have primitive values may not be enough, there may well be attributes which are not to be considered part of the object value. E.g.,

 function MyClass(a, b)
 {
     var c;
     this.getCLazy = function() {
         if (c === undefined) c = a * b // imagine * is really expensive
         return c;
     }
  }

In this above case c is not really important to determine whether any two instances of MyClass are equal, only a and b are important. In some case c might vary between instances and yet not be significant during comparison.

Note this issue applies when members may themselves also be instances of a type and these each would all be required to have a means of determining equality.

Further complicating things is that in Javascript the distinction between data and method is blurred.

An object may reference a method that is to be called as an event handler, this would likely not be considered part of its 'value state'. Whereas another object may well be assigned a function that performs an important calculation and thereby makes this instance different from others simply because it references a different function.

What about an object that has one of its existing prototype methods overridden by another function. Could it still be considered equal to another instance that it otherwise identical? That question can only be answered in each specific case for each type.

As stated earlier the exception would be a strictly typeless object. In which case the only sensible choice is an iterative and recursive comparison of each member. Even then one has to ask what is the 'value' of a function?

Answer 5

I'd advise against hashing or serialization (as the JSON solution suggest). If you need to test if two objects are equal, then you need to define what equals means. It could be that all data members in both objects match, or it could be that must the memory locations match (meaning both variables reference the same object in memory), or may be that only one data member in each object must match.

Recently I developed an object whose constructor creates a new id (starting from 1 and incrementing by 1) each time an instance is created. This object has an isEqual function that compares that id value with the id value of another object and returns true if they match.

In that case I defined "equal" as meaning the the id values match. Given that each instance has a unique id this could be used to enforce the idea that matching objects also occupy the same memory location. Although that is not necessary.

Answer 6

The default equality operator in JavaScript for Objects yields true when they refer to the same location in memory.

var x = {};
var y = {};
var z = x;

x === y; // => false
x === z; // => true

If you require a different equality operator you'll need to add an equals(other) method, or something like it to your classes and the specifics of your problem domain will determine what exactly that means.

Here's a playing card example:

function Card(rank, suit) {
  this.rank = rank;
  this.suit = suit;
  this.equals = function(other) {
     return other.rank == this.rank && other.suit == this.suit;
  };
}

var queenOfClubs = new Card(12, "C");
var kingOfSpades = new Card(13, "S");

queenOfClubs.equals(kingOfSpades); // => false
kingOfSpades.equals(new Card(13, "S")); // => true

Answer 7

I need to mock jQuery POST requests, so the equality that matters to me is that both objects have the same set of properties (none missing in either object), and that each property value is "equal" (according to this definition). I don't care about the objects having mismatching methods.

Here's what I'll be using, it should be good enough for my specific requirements:

function PostRequest() {
    for (var i = 0; i < arguments.length; i += 2) {
        this[arguments[i]] = arguments[i+1];
    }

    var compare = function(u, v) {
        if (typeof(u) != typeof(v)) {
            return false;
        }

        var allkeys = {};
        for (var i in u) {
            allkeys[i] = 1;
        }
        for (var i in v) {
            allkeys[i] = 1;
        }
        for (var i in allkeys) {
            if (u.hasOwnProperty(i) != v.hasOwnProperty(i)) {
                if ((u.hasOwnProperty(i) && typeof(u[i]) == 'function') ||
                    (v.hasOwnProperty(i) && typeof(v[i]) == 'function')) {
                    continue;
                } else {
                    return false;
                }
            }
            if (typeof(u[i]) != typeof(v[i])) {
                return false;
            }
            if (typeof(u[i]) == 'object') {
                if (!compare(u[i], v[i])) {
                    return false;
                }
            } else {
                if (u[i] !== v[i]) {
                    return false;
                }
            }
        }

        return true;
    };

    this.equals = function(o) {
        return compare(this, o);
    };

    return this;
}

Use like so:

foo = new PostRequest('text', 'hello', 'html', '<p>hello</p>');
foo.equals({ html: '<p>hello</p>', text: 'hello' });

Answer 8

Needing a more generic object comparison function than had been posted, I cooked up the following. Critique appreciated...

Object.prototype.equals = function(iObj) {
  if (this.constructor !== iObj.constructor)
    return false;
  var aMemberCount = 0;
  for (var a in this) {
    if (!this.hasOwnProperty(a))
      continue;
    if (typeof this[a] === 'object' && typeof iObj[a] === 'object' ? !this[a].equals(iObj[a]) : this[a] !== iObj[a])
      return false;
    ++aMemberCount;
  }
  for (var a in iObj)
    if (iObj.hasOwnProperty(a))
      --aMemberCount;
  return aMemberCount ? false : true;
}

Answer 9

Why reinvent the wheel? Give underscore.js a try. It has a number of must-have functions such as isEqual().

http://documentcloud.github.com/underscore/#isEqual

_.isEqual(object, other);

It will brute force check each key value - just like the other examples on this page - using es5 and native optimizations if they're available in the browser.

Answer 10

It's useful to consider two objects equal if they have all the same values for all properties and recursively for all nested objects and arrays. I also consider the following two objects equal:

var a = {p1: 1};
var b = {p1: 1, p2: undefined};

Similarly, arrays can have "missing" elements and undefined elements. I would treat those the same as well:

var c = [1, 2];
var d = [1, 2, undefined];

A function that implements this definition of equality:

function isEqual(a, b) {
    if (a === b) {
        return true;
    }

    if (generalType(a) != generalType(b)) {
        return false;
    }

    if (a == b) {
        return true;
    }

    if (typeof a != 'object') {
        return false;
    }

    // null != {}
    if (a instanceof Object != b instanceof Object) {
        return false;
    }

    if (a instanceof Date || b instanceof Date) {
        if (a instanceof Date != b instanceof Date ||
            a.getTime() != b.getTime()) {
            return false;
        }
    }

    var allKeys = [].concat(keys(a), keys(b));
    uniqueArray(allKeys);

    for (var i = 0; i < allKeys.length; i++) {
        var prop = allKeys[i];
        if (!isEqual(a[prop], b[prop])) {
            return false;
        }
    }
    return true;
}

Source code (including the helper functions, generalType and uniqueArray): Unit Test and Test Runner here.