Question

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How do you determine equality for two JavaScript objects?
Object comparison in JavaScript

If I have two arrays or objects and want to compare them, such as

object1 = [
 { shoes:
   [ 'loafer', 'penny' ]
  },
  { beers:
     [ 'budweiser', 'busch' ]
  }
]

object2 = [
 { shoes:
   [ 'loafer', 'penny' ]
  },
  { beers:
     [ 'budweiser', 'busch' ]
  }
]

object1 == object2 // false

this can be annoying if you’re getting a response from a server and trying to see if it’s changed

Answer 1

Update:
In response to the comments and worries surrounding the original suggestion (comparing 2 JSON strings), you could use this function:

function compareObjects(o, p)
{
    var i,
        keysO = Object.keys(o).sort(),
        keysP = Object.keys(p).sort();
    if (keysO.length !== keysP.length)
        return false;//not the same nr of keys
    if (keysO.join('') !== keysP.join(''))
        return false;//different keys
    for (i=0;i<keysO.length;++i)
    {
        if (o[keysO[i]] instanceof Array)
        {
            if (!(p[keysO[i]] instanceof Array))
                return false;
            //if (compareObjects(o[keysO[i]], p[keysO[i]] === false) return false
            //would work, too, and perhaps is a better fit, still, this is easy, too
            if (p[keysO[i]].sort().join('') !== o[keysO[i]].sort().join(''))
                return false;
        }
        else if (o[keysO[i]] instanceof Date)
        {
            if (!(p[keysO[i]] instanceof Date))
                return false;
            if ((''+o[keysO[i]]) !== (''+p[keysO[i]]))
                return false;
        }
        else if (o[keysO[i]] instanceof Function)
        {
            if (!(p[keysO[i]] instanceof Function))
                return false;
            //ignore functions, or check them regardless?
        }
        else if (o[keysO[i]] instanceof Object)
        {
            if (!(p[keysO[i]] instanceof Object))
                return false;
            if (o[keysO[i]] === o)
            {//self reference?
                if (p[keysO[i]] !== p)
                    return false;
            }
            else if (compareObjects(o[keysO[i]], p[keysO[i]]) === false)
                return false;//WARNING: does not deal with circular refs other than ^^
        }
        if (o[keysO[i]] !== p[keysO[i]])//change !== to != for loose comparison
            return false;//not the same value
    }
    return true;
}

But in many cases, it needn’t be that difficult IMO:

JSON.stringify(object1) === JSON.stringify(object2);

If the stringified objects are the same, their values are alike.
For completeness’ sake: JSON simply ignores functions (well, removes them all together). It’s meant to represent Data, not functionality.
Attempting to compare 2 objects that contain only functions will result in true:

JSON.stringify({foo: function(){return 1;}}) === JSON.stringify({foo: function(){ return -1;}});
//evaulutes to:
'{}' === '{}'
//is true, of course

For deep-comparison of objects/functions, you’ll have to turn to libs or write your own function, and overcome the fact that JS objects are all references, so when comparing o1 === ob2 it’ll only return true if both variables point to the same object…

As @a-j pointed out in the comment:

JSON.stringify({a: 1, b: 2}) === JSON.stringify({b: 2, a: 1});

is false, as both stringify calls yield "{"a":1,"b":2}" and "{"b":2,"a":1}" respectively. As to why this is, you need to understand the internals of chrome’s V8 engine. I’m not an expert, and without going into too much detail, here’s what it boils down to:

Each object that is created, and each time it is modified, V8 creates a new hidden C++ class (sort of). If object X has a property a, and another object has the same property, both these JS objects will reference a hidden class that inherits from a shared hidden class that defines this property a. If two objects all share the same basic properties, then they will all reference the same hidden classes, and JSON.stringify will work exactly the same on both objects. That’s a given (More details on V8’s internals here, if you’re interested).

However, in the example pointed out by a-j, both objects are stringified differently. How come? Well, put simply, these objects never exist at the same time:

JSON.stringify({a: 1, b: 2})

This is a function call, an expression that needs to be resolved to the resulting value before it can be compared to the right-hand operand. The second object literal isn’t on the table yet.
The object is stringified, and the exoression is resolved to a string constant. The object literal isn’t being referenced anywhere and is flagged for garbage collection.
After this, the right hand operand (the JSON.stringify({b: 2, a: 1}) expression) gets the same treatment.

All fine and dandy, but what also needs to be taken into consideration is that JS engines now are far more sophisticated than they used to be. Again, I’m no V8 expert, but I think its plausible that a-j’s snippet is being heavily optimized, in that the code is optimized to:

"{"b":2,"a":1}" === "{"a":1,"b":2}"

Essentially omitting the JSON.stringify calls all together, and just adding quotes in the right places. That is, after all, a lot more efficient.