Martin Jespersen Martin Jespersen - 3 months ago 60x
Javascript Question

Generic deep diff between two objects

I have two objects:


The data in
was used to populate a form and
is the result of the user changing data in this form and submitting it.

Both objects are deep, ie. they have properties that are objects or arrays of objects etc - they can be n levels deep, thus the diff algorithm needs to be recursive.

Now I need to not just figure out what was changed (as in added/updated/deleted) from
, but also how to best represent it.

So far my thoughts was to just build a
method that would return an object on the form
but then I thought: somebody else must have needed this before.

So... does anyone know of a library or a piece of code that will do this and maybe have an even better way of representing the difference (in a way that is still JSON serializable)?


I have thought of a better way to represent the updated data, by using the same object structure as
, but turning all property values into objects on the form:

{type: '<update|create|delete>', data: <propertyValue>}

So if
newObj.prop1 = 'new value'
oldObj.prop1 = 'old value'
it would set
returnObj.prop1 = {type: 'update', data: 'new value'}

Update 2:

It gets truely hairy when we get to properties that are arrays, since the array
should be counted as equal to
, which is simple enough for arrays of value based types like string, int & bool, but gets really difficult to handle when it comes to arrays of reference types like objects and arrays.

Example arrays that should be found equal:

[1,[{c: 1},2,3],{a:'hey'}] and [{a:'hey'},1,[3,{c: 1},2]]

Not only is it quite complex to check for this type of deep value equality, but also to figure out a good way to represent the changes that might be.


I wrote a little class that is doing what you want, you can test it here.

Only thing that is different from your proposal is that I don't consider [1,[{c: 1},2,3],{a:'hey'}] and [{a:'hey'},1,[3,{c: 1},2]] to be same, because I think that arrays are not equal if order of their elements is not same. Of course this can be changed if needed. Also this code can be further enhanced to take function as argument that will be used to format diff object in arbitrary way based on passed primitive values (now this job is done by "compareValues" method).

var deepDiffMapper = function() {
    return {
        VALUE_CREATED: 'created',
        VALUE_UPDATED: 'updated',
        VALUE_DELETED: 'deleted',
        VALUE_UNCHANGED: 'unchanged',
        map: function(obj1, obj2) {
            if (this.isFunction(obj1) || this.isFunction(obj2)) {
                throw 'Invalid argument. Function given, object expected.';
            if (this.isValue(obj1) || this.isValue(obj2)) {
                return {type: this.compareValues(obj1, obj2), data: obj1 || obj2};

            var diff = {};
            for (var key in obj1) {
                if (this.isFunction(obj1[key])) {

                var value2 = undefined;
                if ('undefined' != typeof(obj2[key])) {
                    value2 = obj2[key];

                diff[key] =[key], value2);
            for (var key in obj2) {
                if (this.isFunction(obj2[key]) || ('undefined' != typeof(diff[key]))) {

                diff[key] =, obj2[key]);

            return diff;

        compareValues: function(value1, value2) {
            if (value1 === value2) {
                return this.VALUE_UNCHANGED;
            if ('undefined' == typeof(value1)) {
                return this.VALUE_CREATED;
            if ('undefined' == typeof(value2)) {
                return this.VALUE_DELETED;

            return this.VALUE_UPDATED;
        isFunction: function(obj) {
            return {}.toString.apply(obj) === '[object Function]';
        isArray: function(obj) {
            return {}.toString.apply(obj) === '[object Array]';
        isObject: function(obj) {
            return {}.toString.apply(obj) === '[object Object]';
        isValue: function(obj) {
            return !this.isObject(obj) && !this.isArray(obj);

var result ={
      a:'i am unchanged',
      b:'i am deleted',
      e:{ a: 1,b:false, c: null},
      f: [1,{a: 'same',b:[{a:'same'},{d: 'delete'}]}]
      a:'i am unchanged',
      c:'i am created',
      e:{ a: '1', b: '', d:'created'},
      f: [{a: 'same',b:[{a:'same'},{c: 'create'}]},1]