JSON object != JavaScript object?

Question

For convenience I wrote a simple toJSON prototype, for handling JSON that I know to be safe:

String.prototype.toJSON = function () {
    return JSON.parse(this.valueOf());
};

I am using it in testing my web-services. Unfortunately even with this simple test:

var v0 = '{"echo":"hello_world"}'.toJSON(), v1 = {"echo": "hello_world"};

It fails:

console.log(v0 == v1);            // false
console.log(v0 === v1);           // false
console.log(v0.echo == v1.echo);  // true
console.log(v0.echo === v1.echo); // true

What do I not know about JavaScript which is causing this issue?

see: http://stackoverflow.com/questions/201183/how-do-you-determine-equality-for-two-javascript-objects — Kevin Bowersox, Dec 22 '13 at 11:43
Possible duplication: http://stackoverflow.com/questions/1068834/object-comparison-in-javascript — Eraden, Dec 22 '13 at 11:43
`{a:1} !== {a:1}`. Two objects are the same... if they are the same one, and point to the same object in the heap. — bgusach, Dec 22 '13 at 11:51

score 3 · Answer 1 · answered Dec 22 '13 at 11:42

3

Just because you have the same content, that does not mean that you have the same object instance. If you had done v1 = v0 instead of initializing v1 seperatly the first two would have returned true.

Update: If you need to compare the two instances to find if they have equal content then you need to define a function which compares each member.

answered Dec 22 '13 at 11:42

fredrik

6,483
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I am getting `v0` from an HTTP GET request; and have a test which ensures that what I get from the HTTP GET matches my expectation. – A T Dec 22 '13 at 11:44

Oleg Belousov · Accepted Answer · 2013-12-22T11:54:43.887

An object in JavaScript, just like everything else except primitives(int, string, Boolean) is a reference.

Having 2 different duplicate objects, means having 2 different references that point to different places within the hype.

You can implement something as simple as that, to basically iterate over all of the primitive properties of an object, and compare them one by one:

Object.prototype.equals = function(x)
{
    for(p in this)
    {
        switch(typeof(this[p]))
        {
            case 'object':
                if (!this[p].equals(x[p])) { return false }; break;
            case 'function':
                if (typeof(x[p])=='undefined' || (p != 'equals' && this[p].toString() != x[p].toString())) { return false; }; break;
            default:
                if (this[p] != x[p]) { return false; }
        }
    }

    for(p in x)
    {
        if(typeof(this[p])=='undefined') {return false;}
    }

    return true;
}

Thanks, this looks rather simple. Is the [stack-based underscore one](http://underscorejs.org/underscore.js) higher performance? — A T, Dec 22 '13 at 12:03
Both of the methods conduct a recursive compression, generally speaking the time complexity is log(n) in both cases. — Oleg Belousov, Dec 22 '13 at 12:06

score 1 · Answer 3 · answered Dec 22 '13 at 11:44

1

For two objects to be == they must be the same instance. v0 and v1 are two different instances. If you want to do a deep comparison of objects you can use something like underscore's isEqual method: http://underscorejs.org/#isEqual

_.isEqual(v0, v1);

answered Dec 22 '13 at 11:44

Hallvar Helleseth

1,867
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Oh okay, didn't know that a deep comparison operator wasn't supported out-of-the-box in JavaScript. Will consider using, e.g. [this](http://stackoverflow.com/a/13143059/587021). – A T Dec 22 '13 at 11:46
It is not about deep comparison. Two objects are equal if they are the same object. One thing is comparing the object, and another one is to compare the value (which is what _.isEqual does). – bgusach Dec 22 '13 at 11:52
Thanks, I was easily able to get underscore to work within my scope. – A T Dec 22 '13 at 12:01
From underscores API docs on isEqual: Performs an optimized deep comparison between the two objects, to determine if they should be considered equal. – Hallvar Helleseth Dec 22 '13 at 12:11

JSON object != JavaScript object?

3 Answers3