How to determine equality for two JavaScript objects?

The Question :

727 people think this question is useful

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

Stack Overflow question Is there any kind of hashCode function in JavaScript? is similar to this question, but requires a more academic answer. The scenario above demonstrates why it would be necessary to have one, and I’m wondering if there is any equivalent solution.

The Question Comments :
  • Also look into this question stackoverflow.com/q/1068834/1671639
  • Note that, even in Java, a.hashCode() == b.hashCode() does not imply that a is equal to b. It’s a necessary condition, not a sufficient one.
  • If you HAVE to compare objects in your code than you are probably writing your code wrong. The better question might be: “How can I write this code so I don’t have to compare objects?”
  • Try object-equals package.
  • @th317erd can you please explain yourself?…

The Answer 1

191 people think this answer is useful

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 they are equal at a value level. However, it is up to the specific type to define how an Equals method should be implemented. An iterative comparison 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. For example,

 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 cases 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, and 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?

The Answer 2

536 people think this answer is useful

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

_.isEqual(object, other);

It will brute force check each key value – just like the other examples on this page – using ECMAScript 5 and native optimizations if they’re available in the browser.

Note: Previously this answer recommended Underscore.js, but lodash has done a better job of getting bugs fixed and addressing issues with consistency.

The Answer 3

166 people think this answer is useful

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

The Answer 4

84 people think this answer is useful

If you are working in AngularJS, the angular.equals function will determine if two objects are equal. In Ember.js use isEqual.

  • angular.equals – See the docs or source for more on this method. It does a deep compare on arrays too.
  • Ember.js isEqual – See the docs or source for more on this method. It does not do a deep compare on arrays.

var purple = [{"purple": "drank"}];
var drank = [{"purple": "drank"}];

if(angular.equals(purple, drank)) {
    document.write('got dat');
}
<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.4.5/angular.min.js"></script>

The Answer 5

74 people think this answer is useful

This is my version. It is using new Object.keys feature that is introduced in ES5 and ideas/tests from +, + and +:

function objectEquals(x, y) {
    'use strict';

    if (x === null || x === undefined || y === null || y === undefined) { return x === y; }
    // after this just checking type of one would be enough
    if (x.constructor !== y.constructor) { return false; }
    // if they are functions, they should exactly refer to same one (because of closures)
    if (x instanceof Function) { return x === y; }
    // if they are regexps, they should exactly refer to same one (it is hard to better equality check on current ES)
    if (x instanceof RegExp) { return x === y; }
    if (x === y || x.valueOf() === y.valueOf()) { return true; }
    if (Array.isArray(x) &amp;&amp; x.length !== y.length) { return false; }

    // if they are dates, they must had equal valueOf
    if (x instanceof Date) { return false; }

    // if they are strictly equal, they both need to be object at least
    if (!(x instanceof Object)) { return false; }
    if (!(y instanceof Object)) { return false; }

    // recursive object equality check
    var p = Object.keys(x);
    return Object.keys(y).every(function (i) { return p.indexOf(i) !== -1; }) &amp;&amp;
        p.every(function (i) { return objectEquals(x[i], y[i]); });
}


///////////////////////////////////////////////////////////////
/// The borrowed tests, run them by clicking "Run code snippet"
///////////////////////////////////////////////////////////////
var printResult = function (x) {
    if (x) { document.write('<div style="color: green;">Passed</div>'); }
    else { document.write('<div style="color: red;">Failed</div>'); }
};
var assert = { isTrue: function (x) { printResult(x); }, isFalse: function (x) { printResult(!x); } }
assert.isTrue(objectEquals(null,null));
assert.isFalse(objectEquals(null,undefined));
assert.isFalse(objectEquals(/abc/, /abc/));
assert.isFalse(objectEquals(/abc/, /123/));
var r = /abc/;
assert.isTrue(objectEquals(r, r));

assert.isTrue(objectEquals("hi","hi"));
assert.isTrue(objectEquals(5,5));
assert.isFalse(objectEquals(5,10));

assert.isTrue(objectEquals([],[]));
assert.isTrue(objectEquals([1,2],[1,2]));
assert.isFalse(objectEquals([1,2],[2,1]));
assert.isFalse(objectEquals([1,2],[1,2,3]));

assert.isTrue(objectEquals({},{}));
assert.isTrue(objectEquals({a:1,b:2},{a:1,b:2}));
assert.isTrue(objectEquals({a:1,b:2},{b:2,a:1}));
assert.isFalse(objectEquals({a:1,b:2},{a:1,b:3}));

assert.isTrue(objectEquals({1:{name:"mhc",age:28}, 2:{name:"arb",age:26}},{1:{name:"mhc",age:28}, 2:{name:"arb",age:26}}));
assert.isFalse(objectEquals({1:{name:"mhc",age:28}, 2:{name:"arb",age:26}},{1:{name:"mhc",age:28}, 2:{name:"arb",age:27}}));

Object.prototype.equals = function (obj) { return objectEquals(this, obj); };
var assertFalse = assert.isFalse,
    assertTrue = assert.isTrue;

assertFalse({}.equals(null));
assertFalse({}.equals(undefined));

assertTrue("hi".equals("hi"));
assertTrue(new Number(5).equals(5));
assertFalse(new Number(5).equals(10));
assertFalse(new Number(1).equals("1"));

assertTrue([].equals([]));
assertTrue([1,2].equals([1,2]));
assertFalse([1,2].equals([2,1]));
assertFalse([1,2].equals([1,2,3]));
assertTrue(new Date("2011-03-31").equals(new Date("2011-03-31")));
assertFalse(new Date("2011-03-31").equals(new Date("1970-01-01")));

assertTrue({}.equals({}));
assertTrue({a:1,b:2}.equals({a:1,b:2}));
assertTrue({a:1,b:2}.equals({b:2,a:1}));
assertFalse({a:1,b:2}.equals({a:1,b:3}));

assertTrue({1:{name:"mhc",age:28}, 2:{name:"arb",age:26}}.equals({1:{name:"mhc",age:28}, 2:{name:"arb",age:26}}));
assertFalse({1:{name:"mhc",age:28}, 2:{name:"arb",age:26}}.equals({1:{name:"mhc",age:28}, 2:{name:"arb",age:27}}));

var a = {a: 'text', b:[0,1]};
var b = {a: 'text', b:[0,1]};
var c = {a: 'text', b: 0};
var d = {a: 'text', b: false};
var e = {a: 'text', b:[1,0]};
var i = {
    a: 'text',
    c: {
        b: [1, 0]
    }
};
var j = {
    a: 'text',
    c: {
        b: [1, 0]
    }
};
var k = {a: 'text', b: null};
var l = {a: 'text', b: undefined};

assertTrue(a.equals(b));
assertFalse(a.equals(c));
assertFalse(c.equals(d));
assertFalse(a.equals(e));
assertTrue(i.equals(j));
assertFalse(d.equals(k));
assertFalse(k.equals(l));

// from comments on stackoverflow post
assert.isFalse(objectEquals([1, 2, undefined], [1, 2]));
assert.isFalse(objectEquals([1, 2, 3], { 0: 1, 1: 2, 2: 3 }));
assert.isFalse(objectEquals(new Date(1234), 1234));

// no two different function is equal really, they capture their context variables
// so even if they have same toString(), they won't have same functionality
var func = function (x) { return true; };
var func2 = function (x) { return true; };
assert.isTrue(objectEquals(func, func));
assert.isFalse(objectEquals(func, func2));
assert.isTrue(objectEquals({ a: { b: func } }, { a: { b: func } }));
assert.isFalse(objectEquals({ a: { b: func } }, { a: { b: func2 } }));

The Answer 6

53 people think this answer is useful

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));

NOTE: While this answer will work in many cases, as several people have pointed out in the comments it’s problematic for a variety of reasons. In pretty much all cases you’ll want to find a more robust solution.

The Answer 7

49 people think this answer is useful

Short functional deepEqual implementation:

function deepEqual(x, y) {
  return (x &amp;&amp; y &amp;&amp; typeof x === 'object' &amp;&amp; typeof y === 'object') ?
    (Object.keys(x).length === Object.keys(y).length) &amp;&amp;
      Object.keys(x).reduce(function(isEqual, key) {
        return isEqual &amp;&amp; deepEqual(x[key], y[key]);
      }, true) : (x === y);
}

Edit: version 2, using jib’s suggestion and ES6 arrow functions:

function deepEqual(x, y) {
  const ok = Object.keys, tx = typeof x, ty = typeof y;
  return x &amp;&amp; y &amp;&amp; tx === 'object' &amp;&amp; tx === ty ? (
    ok(x).length === ok(y).length &amp;&amp;
      ok(x).every(key => deepEqual(x[key], y[key]))
  ) : (x === y);
}

The Answer 8

23 people think this answer is useful

In Node.js, you can use its native require("assert").deepStrictEqual. More info: http://nodejs.org/api/assert.html

For example:

var assert = require("assert");
assert.deepStrictEqual({a:1, b:2}, {a:1, b:3}); // will throw AssertionError

Another example that returns true / false instead of returning errors:

var assert = require("assert");

function deepEqual(a, b) {
    try {
      assert.deepEqual(a, b);
    } catch (error) {
      if (error.name === "AssertionError") {
        return false;
      }
      throw error;
    }
    return true;
};

The Answer 9

22 people think this answer is useful

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.

The Answer 10

22 people think this answer is useful

If you have a deep copy function handy, you can use the following trick to still use JSON.stringify while matching the order of properties:

function equals(obj1, obj2) {
    function _equals(obj1, obj2) {
        return JSON.stringify(obj1)
            === JSON.stringify($.extend(true, {}, obj1, obj2));
    }
    return _equals(obj1, obj2) &amp;&amp; _equals(obj2, obj1);
}

Demo: http://jsfiddle.net/CU3vb/3/

Rationale:

Since the properties of obj1 are copied to the clone one by one, their order in the clone will be preserved. And when the properties of obj2 are copied to the clone, since properties already existing in obj1 will simply be overwritten, their orders in the clone will be preserved.

The Answer 11

20 people think this answer is useful

Simplest and logical solutions for comparing everything Like Object, Array, String, Int…

JSON.stringify({a: val1}) === JSON.stringify({a: val2})

Note:

  • you need to replace val1and val2 with your Object
  • for the object, you have to sort(by key) recursively for both side objects

The Answer 12

13 people think this answer is useful

I use this comparable function to produce copies of my objects that are JSON comparable:

var comparable = o => (typeof o != 'object' || !o)? o :
  Object.keys(o).sort().reduce((c, key) => (c[key] = comparable(o[key]), c), {});

// Demo:

var a = { a: 1, c: 4, b: [2, 3], d: { e: '5', f: null } };
var b = { b: [2, 3], c: 4, d: { f: null, e: '5' }, a: 1 };

console.log(JSON.stringify(comparable(a)));
console.log(JSON.stringify(comparable(b)));
console.log(JSON.stringify(comparable(a)) == JSON.stringify(comparable(b)));
<div id="div"></div>

Comes in handy in tests (most test frameworks have an is function). E.g.

is(JSON.stringify(comparable(x)), JSON.stringify(comparable(y)), 'x must match y');

If a difference is caught, strings get logged, making differences spottable:

x must match y
got      {"a":1,"b":{"0":2,"1":3},"c":7,"d":{"e":"5","f":null}},
expected {"a":1,"b":{"0":2,"1":3},"c":4,"d":{"e":"5","f":null}}.

The Answer 13

12 people think this answer is useful

For those of you using NodeJS, there is a convenient method called isDeepStrictEqual on the native Util library that can achieve this.

const util = require('util');

const obj1 = {
  foo: "bar",
  baz: [1, 2]
};

const obj2 = {
  foo: "bar",
  baz: [1, 2]
};


obj1 == obj2 // false
util.isDeepStrictEqual(obj1, obj2) // true


https://nodejs.org/api/util.html#util_util_isdeepstrictequal_val1_val2

The Answer 14

11 people think this answer is useful

Heres’s a solution in ES6/ES2015 using a functional-style approach:

const typeOf = x => 
  ({}).toString
      .call(x)
      .match(/\[object (\w+)\]/)[1]

function areSimilar(a, b) {
  const everyKey = f => Object.keys(a).every(f)

  switch(typeOf(a)) {
    case 'Array':
      return a.length === b.length &amp;&amp;
        everyKey(k => areSimilar(a.sort()[k], b.sort()[k]));
    case 'Object':
      return Object.keys(a).length === Object.keys(b).length &amp;&amp;
        everyKey(k => areSimilar(a[k], b[k]));
    default:
      return a === b;
  }
}

demo available here

The Answer 15

10 people think this answer is useful

I don’t know if anyone’s posted anything similar to this, but here’s a function I made to check for object equalities.

function objectsAreEqual(a, b) {
  for (var prop in a) {
    if (a.hasOwnProperty(prop)) {
      if (b.hasOwnProperty(prop)) {
        if (typeof a[prop] === 'object') {
          if (!objectsAreEqual(a[prop], b[prop])) return false;
        } else {
          if (a[prop] !== b[prop]) return false;
        }
      } else {
        return false;
      }
    }
  }
  return true;
}

Also, it’s recursive, so it can also check for deep equality, if that’s what you call it.

The Answer 16

7 people think this answer is useful
var object1 = {name: "humza" , gender : "male", age: 23}
var object2 = {name: "humza" , gender : "male", age: 23}
var result = Object.keys(object1).every((key) =>  object1[key] === object2[key])

Result will be true if object1 has same values on object2.

The Answer 17

7 people think this answer is useful

This question has more than 30 answers already. I am going to summarize and explain them (with a “my father” analogy) and add my suggested solution.

You have 4+1 classes of solutions:


1) Use a hacky incomplete quick one-liner

Good if you are in a rush and 99% correctness works.

Examples of this is, JSON.stringify() suggested by Pratik Bhalodiya, or JSON.encode by Joel Anair, or .toString(), or other methods that transform your objects into a String and then compare the two Strings using === character by character.

The drawback, however, is that there is no globally standard unique representation of an Object in String. e.g. { a: 5, b: 8} and {b: 8 and a: 5 } are equal.

  • Pros: Fast, quick.
  • Cons: Hopefully works! It will not work if the environment/browser/engine memorizes the ordering for objects (e.g. Chrome/V8) and the order of the keys are different (Thanks to Eksapsy.) So, not guaranteed at all. Performance wouldn’t be great either in large objects.

My Father Analogy

When I am talking about my father, “my tall handsome father” and “my handsome tall father” are the same person! But the two strings are not the same.

Note that there is actually a correct (standard way) order of adjectives in English grammar, which says it should be a “handsome tall man,” but you are risking your competency if you blindly assume Javascript engine of iOS 8 Safari is also abiding the same grammar, blindly! #WelcomeToJavascriptNonStandards


2) Write your own DIY recursive function

Good if you are learning.

Examples are atmin’s solution.

The biggest disadvantage is you will definitely miss some edge cases. Have you considered a self-reference in object values? Have you considered NaN? Have you considered two objects that have the same ownProperties but different prototypical parents?

I would only encourage people to do this if they are practicing and the code is not going to go in production. That’s the only case that reinventing the wheel has justifications.

  • Pros: Learning opportunity.
  • Cons: Not reliable. Takes time and concerns.

My Father Analogy

It’s like assuming if my dad’s name is “John Smith” and his birthday is “1/1/1970”, then anyone whose name is “John Smith” and is born on “1/1/1970” is my father.

That’s usually the case, but what if there are two “John Smith”s born on that day? If you think you will consider their height, then that’s increasing the accuracy but still not a perfect comparison.

2.1 You limited scope DIY comparator

Rather than going on a wild chase of checking all the properties recursively, one might consider checking only “a limited” number of properties. For instance, if the objects are Users, you can compare their emailAddress field.

It’s still not a perfect one, but the benefits over solution #2 are:

  1. It’s predictable, and it’s less likely to crash.
  2. You are driving the “definition” of equality, rather than relying on a wild form and shape of the Object and its prototype and nested properties.

3) Use a library version of equal function

Good if you need a production-level quality, and you cannot change the design of the system.

Examples are _.equal of lodash, already in coolaj86’s answer or Angular’s or Ember’s as mentioned in Tony Harvey’s answer or Node’s by Rafael Xavier.

  • Pros: It’s what everyone else does.
  • Cons: External dependency, which can cost you extra memory/CPU/Security concerns, even a little bit. Also, can still miss some edge cases (e.g. whether two objects having same ownProperties but different prototypical parents should be considered the same or not.) Finally, you might be unintentionally band-aiding an underlying design problem with this; just saying!

My Father Analogy

It’s like paying an agency to find my biological father, based on his phone, name, address, etc.

It’s gonna cost more, and it’s probably more accurate than myself running the background check, but doesn’t cover edge cases like when my father is immigrant/asylum and his birthday is unknown!


4) Use an IDentifier in the Object

Good if you [still] can change the design of the system (objects you are dealing with) and you want your code to last long.

It’s not applicable in all cases, and might not be very performant. However, it’s a very reliable solution, if you can make it.

The solution is, every object in the system will have a unique identifier along with all the other properties. The uniqueness of the identifier will be guaranteed at the time of generation. And you will use this ID (also known as UUID/GUID — Globally/Universally Unique Identifier) when it comes to comparing two objects. i.e. They are equal if and only if these IDs are equal.

The IDs can be simple auto_incremental numbers, or a string generated via a library (advised) or a piece of code. All you need to do is make sure it’s always unique, which in case of auto_incremental it can be built-in, or in case of UUID, can be checked will all existing values (e.g. MySQL’s UNIQUE column attribute) or simply (if coming from a library) be relied upon giving the extremely low likelihood of a collision.

Note that you also need to store the ID with the object at all times (to guarantee its uniqueness), and computing it in real-time might not be the best approach.

  • Pros: Reliable, efficient, not dirty, modern.
  • Cons: Needs extra space. Might need a redesign of the system.

My Father Analogy

It’s like known my father’s Social Security Number is 911-345-9283, so anyone who has this SSN is my father, and anyone who claims to be my father must have this SSN.


Conclusion

I personally prefer solution #4 (ID) over them all for accuracy and reliability. If it’s not possible I’d go with #2.1 for predictability, and then #3. If neither is possible, #2 and finally #1.

The Answer 18

6 people think this answer is useful

ES6: The minimum code I could get it done, is this. It do deep comparison recursively by stringifying all key value array sorted representing the object, the only limitation is no methods or symbols are compare.

const compareObjects = (a, b) => { 
  let s = (o) => Object.entries(o).sort().map(i => { 
     if(i[1] instanceof Object) i[1] = s(i[1]);
     return i 
  }) 
  return JSON.stringify(s(a)) === JSON.stringify(s(b))
}

console.log(compareObjects({b:4,a:{b:1}}, {a:{b:1},b:4}));

IMPORTANT: This function is doing a JSON.stringfy in an ARRAY with the keys sorted and NOT in the object it self:

  1. [“a”, [“b”, 1]]
  2. [“b”, 4]

The Answer 19

5 people think this answer is useful

you can use _.isEqual(obj1, obj2) from the underscore.js library.

Here is an example:

var stooge = {name: 'moe', luckyNumbers: [13, 27, 34]};
var clone  = {name: 'moe', luckyNumbers: [13, 27, 34]};
stooge == clone;
=> false
_.isEqual(stooge, clone);
=> true

See the official documentation from here: http://underscorejs.org/#isEqual

The Answer 20

4 people think this answer is useful

A simple solution to this issue that many people don’t realize is to sort the JSON strings (per character). This is also usually faster than the other solutions mentioned here:

function areEqual(obj1, obj2) {
    var a = JSON.stringify(obj1), b = JSON.stringify(obj2);
    if (!a) a = '';
    if (!b) b = '';
    return (a.split('').sort().join('') == b.split('').sort().join(''));
}

Another useful thing about this method is you can filter comparisons by passing a “replacer” function to the JSON.stringify functions (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify#Example_of_using_replacer_parameter). The following will only compare all objects keys that are named “derp”:

function areEqual(obj1, obj2, filter) {
    var a = JSON.stringify(obj1, filter), b = JSON.stringify(obj2, filter);
    if (!a) a = '';
    if (!b) b = '';
    return (a.split('').sort().join('') == b.split('').sort().join(''));
}
var equal = areEqual(obj1, obj2, function(key, value) {
    return (key === 'derp') ? value : undefined;
});

The Answer 21

4 people think this answer is useful

Just wanted to contribute my version of objects comparison utilizing some es6 features. It doesn’t take an order into account. After converting all if/else’s to ternary I’ve came with following:

function areEqual(obj1, obj2) {

    return Object.keys(obj1).every(key => {

            return obj2.hasOwnProperty(key) ?
                typeof obj1[key] === 'object' ?
                    areEqual(obj1[key], obj2[key]) :
                obj1[key] === obj2[key] :
                false;

        }
    )
}

The Answer 22

4 people think this answer is useful

Assuming that the order of the properties in the object is not changed.

JSON.stringify() works for deep and non-deep both types of objects, not very sure of performance aspects:

var object1 = {
  key: "value"
};

var object2 = {
  key: "value"
};

var object3 = {
  key: "no value"
};

console.log('object1 and object2 are equal: ', JSON.stringify(object1) === JSON.stringify(object2));

console.log('object2 and object3 are equal: ', JSON.stringify(object2) === JSON.stringify(object3));

The Answer 23

3 people think this answer is useful

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' &amp;&amp; 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;
}

The Answer 24

3 people think this answer is useful

If you are comparing JSON objects you can use https://github.com/mirek/node-rus-diff

npm install rus-diff

Usage:

a = {foo:{bar:1}}
b = {foo:{bar:1}}
c = {foo:{bar:2}}

var rusDiff = require('rus-diff').rusDiff

console.log(rusDiff(a, b)) // -> false, meaning a and b are equal
console.log(rusDiff(a, c)) // -> { '$set': { 'foo.bar': 2 } }

If two objects are different, a MongoDB compatible {$rename:{...}, $unset:{...}, $set:{...}} like object is returned.

The Answer 25

3 people think this answer is useful

I faced the same problem and deccided to write my own solution. But because I want to also compare Arrays with Objects and vice-versa, I crafted a generic solution. I decided to add the functions to the prototype, but one can easily rewrite them to standalone functions. Here is the code:

Array.prototype.equals = Object.prototype.equals = function(b) {
    var ar = JSON.parse(JSON.stringify(b));
    var err = false;
    for(var key in this) {
        if(this.hasOwnProperty(key)) {
            var found = ar.find(this[key]);
            if(found > -1) {
                if(Object.prototype.toString.call(ar) === "[object Object]") {
                    delete ar[Object.keys(ar)[found]];
                }
                else {
                    ar.splice(found, 1);
                }
            }
            else {
                err = true;
                break;
            }
        }
    };
    if(Object.keys(ar).length > 0 || err) {
        return false;
    }
    return true;
}

Array.prototype.find = Object.prototype.find = function(v) {
    var f = -1;
    for(var i in this) {
        if(this.hasOwnProperty(i)) {
            if(Object.prototype.toString.call(this[i]) === "[object Array]" || Object.prototype.toString.call(this[i]) === "[object Object]") {
                if(this[i].equals(v)) {
                    f = (typeof(i) == "number") ? i : Object.keys(this).indexOf(i);
                }
            }
            else if(this[i] === v) {
                f = (typeof(i) == "number") ? i : Object.keys(this).indexOf(i);
            }
        }
    }
    return f;
}

This Algorithm is split into two parts; The equals function itself and a function to find the numeric index of a property in an array / object. The find function is only needed because indexof only finds numbers and strings and no objects .

One can call it like this:

({a: 1, b: "h"}).equals({a: 1, b: "h"});

The function either returns true or false, in this case true. The algorithm als allows comparison between very complex objects:

({a: 1, b: "hello", c: ["w", "o", "r", "l", "d", {answer1: "should be", answer2: true}]}).equals({b: "hello", a: 1, c: ["w", "d", "o", "r", {answer1: "should be", answer2: true}, "l"]})

The upper example will return true, even tho the properties have a different ordering. One small detail to look out for: This code also checks for the same type of two variables, so “3” is not the same as 3.

The Answer 26

2 people think this answer is useful

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.

The Answer 27

2 people think this answer is useful

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.

The Answer 28

2 people think this answer is useful

I’m making the following assumptions with this function:

  1. You control the objects you are comparing and you only have primitive values (ie. not nested objects, functions, etc.).
  2. Your browser has support for Object.keys.

This should be treated as a demonstration of a simple strategy.

/**
 * Checks the equality of two objects that contain primitive values. (ie. no nested objects, functions, etc.)
 * @param {Object} object1
 * @param {Object} object2
 * @param {Boolean} [order_matters] Affects the return value of unordered objects. (ex. {a:1, b:2} and {b:2, a:1}).
 * @returns {Boolean}
 */
function isEqual( object1, object2, order_matters ) {
    var keys1 = Object.keys(object1),
        keys2 = Object.keys(object2),
        i, key;

    // Test 1: Same number of elements
    if( keys1.length != keys2.length ) {
        return false;
    }

    // If order doesn't matter isEqual({a:2, b:1}, {b:1, a:2}) should return true.
    // keys1 = Object.keys({a:2, b:1}) = ["a","b"];
    // keys2 = Object.keys({b:1, a:2}) = ["b","a"];
    // This is why we are sorting keys1 and keys2.
    if( !order_matters ) {
        keys1.sort();
        keys2.sort();
    }

    // Test 2: Same keys
    for( i = 0; i < keys1.length; i++ ) {
        if( keys1[i] != keys2[i] ) {
            return false;
        }
    }

    // Test 3: Values
    for( i = 0; i < keys1.length; i++ ) {
        key = keys1[i];
        if( object1[key] != object2[key] ) {
            return false;
        }
    }

    return true;
}

The Answer 29

2 people think this answer is useful

This is an addition for all the above, not a replacement. If you need to fast shallow-compare objects without need to check extra recursive cases. Here is a shot.

This compares for: 1) Equality of number of own properties, 2) Equality of key names, 3) if bCompareValues == true, Equality of corresponding property values and their types (triple equality)

var shallowCompareObjects = function(o1, o2, bCompareValues) {
    var s, 
        n1 = 0,
        n2 = 0,
        b  = true;

    for (s in o1) { n1 ++; }
    for (s in o2) { 
        if (!o1.hasOwnProperty(s)) {
            b = false;
            break;
        }
        if (bCompareValues &amp;&amp; o1[s] !== o2[s]) {
            b = false;
            break;
        }
        n2 ++;
    }
    return b &amp;&amp; n1 == n2;
}

The Answer 30

2 people think this answer is useful

For comparing keys for simple key/value pairs object instances, I use:

function compareKeys(r1, r2) {
    var nloops = 0, score = 0;
    for(k1 in r1) {
        for(k2 in r2) {
            nloops++;
            if(k1 == k2)
                score++; 
        }
    }
    return nloops == (score * score);
};

Once keys are compared, a simple additional for..in loop is enough.

Complexity is O(N*N) with N is the number of keys.

I hope/guess objects I define won’t hold more than 1000 properties…

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