Get the delta of two javascript objects

Question

I have two large, nested javascript objects and I would like to compare them and create an object that represents only the differences. I intend to use this to create a PATCH request.

Given oldObj and newObj:

• Properties that are only on newObj should be in the diff
• Properties that are only on oldObj should be in the diff
• Properties that are on both objects should use the value from newObj if the value is an array, string, or number
• The objects should be recursively compared
• No need to merge arrays fancily, complete replace is ok

This may look like a duplicate, but I don't think it is. This solution (1) is one level deep only (the answer below is non-recursive, blows up on arrays, and is not bi-directional). this solution (2) returns unchanged properties is not bi-directional.

Target input/output:

diff({a:1},{a:0}); // {a:0}

diff({a:1},{b:1}); // {a:1,b:1}

diff({
  a: { x: 1 },
  b: 1
},
{
  a: { x: 0 },
  b: 1
}) // {a:{x:0}}

diff({a:[1,3,5,7]},{a:[1,3,7]}); // {a:[1,3,7]} 

I am using the following method which is modified from solution 1. It meets all criteria except diff({a:1},{b:1}) // {a:1,b:1} because it only compares in one direction.

jsonDiff = function(oldObject, newObject) {
  var diff, i, innerDiff;
  diff = {};
  for (i in newObject) {
    innerDiff = {};
    if (_.isArray(newObject[i])) {
      if (!_.isEqual(newObject[i], oldObject[i])) {
        diff[i] = newObject[i];
      }
    } else if (typeof newObject[i] === 'object') {
      innerDiff = jsonDiff(oldObject[i], newObject[i]);
      if (!_.isEmpty(innerDiff)) {
        diff[i] = innerDiff;
      }
    } else if (!oldObject) {
      diff[i] = newObject[i];
    } else if (!oldObject.hasOwnProperty(i)) {
      diff[i] = newObject[i];
    } else if (oldObject[i] !== newObject[i]) {
      diff[i] = newObject[i];
    }
  }
  return diff;
};

I have seen the jsonDiffPatch library, but I do not need all the metadata it creates, just the raw diff object. Is there a mini library that just does this? Seems sort of necessary to implement PATCH nicely, but I can't find one. Anyone have a small gist for this?

Answer 1

Here is a function that should work for you, more comments than code:

// diffObjs: return differences between JavaScript values
//
// Function:
//
//    Compare two JavaScript values, and return a two-element
//    array that contains a minimal representation of the difference
//    between the two.
//
//    Values may be scalar (e.g., string, integer, boolean) or objects,
//    including arrays.  When the two values match exactly, that is,
//    if the '===' operator between the two would return 'true', we return NULL.
//    
//    When the result contains an object or array, only data, not references,
//    are copied from the arguments.  This makes for a large size result
//    but one whose manipulation will not affect the original arguments.
//
// Args:
//    v1, v2: values to compare
//
// Specific behaviors:
//
//    *Return NULL if v1 === v2*
//
//    This happens when two scalar (non-object) values match, or when the same
//    object or array is passed in both arguments.
//    e.g.,
//        
//        var my_obj = { member1: 0, member1: 'dog' };
//        var my_array = [ 1, 'cat' ];
//        var my_int = 7;
//        var no_val = null;
//
//        diffObjs(my_int, my_int)        ==> NULL
//        diffObjs(1, 1)                  ==> NULL
//        diffObjs(my_obj, my_obj)        ==> NULL
//        diffObjs({x:1,y:2}, {x:1,y:2})  ==> NULL
//        diffObjs(my_array, my_array)    ==> NULL
//        diffObjs([1,'a'], [1,'1'])      ==> NULL
//        diffObjs(null, null)            ==> NULL
//        diffObjs(no_val, null)          ==> NULL
//
//    *Return copies of v1 and v2 on type mismatch*:
//
//    When type of v1 and v2 are different or one is an array and the other
//    is an object, the result array will contain exect copies of both
//    v1 and v2.
//
//    *Return minimal representation of differences among non-array objects*:
//
//    Otherwise, when two objects are passed in, element 0
//    in the result array contains the members and their values
//    that exist in v1 but not v2, or members that exist in both
//    v1 and v2 that have different values.  Element 1 contains
//    the same but with respect to v2, that is members and their
//    values that exist in v2 but not v1, or members that exist in
//    both v1 and v2 that have different values.
//    
//    Note: The members are represented in the result objects only when
//    they are specific to the object of the corresponding value argument
//    or when the members exist in both and have different values.  The
//    caller therefore can tell whether the object mismatch exists 
//    because of specificity of a member to one object vs. a mismatch
//    in values where one is null and the other is not.
//
//    Examples:
//        diffObjs({a:10, b:"dog"}, {a:1, b:"dog"}    ==> [ {a:10}, {a:1} ]
//        diffObjs({a:10},          {a:10, b:"dog"}   ==> [ {}, {b:"dog"} ]
//        diffObjs({a:10, c:null},  {a:10, b:"dog"}   ==> [ {c:null}, {b:"dog"} ]
//        diffObjs({a:[1], b:"cat"},{a:1, b:"dog"}    ==> [ {a:[1], b:"cat"}, {a:1, b:"dog"} ]
//        diffObjs(
//            {a:{ m1:"x", m2:"y"}, b:3 },
//            {a:{ m1:"x", m2:"z", m3:1 }, b:3 } )    ==> [ {a:{m2:"y"}}, {a:{m2:"z",m3:1}} ]
//
//    *Return copies of compared arrays when differing by position or value*
//
//    If the two arguments arrays, the results in elements 0 and 1
//    will contain results in array form that do not match with respect
//    to both value and order.  If two positionally corresponding
//    elements in the array arguments have identical value (e.g., two
//    scalars with matching values or two references to the same object), 
//    the corresponding values in the array will be null.  The
//    cardinality of the arrays within the result array will therefore
//    always match that of the corresponding arguments.
//
//    Examples:
//        diffObjs([1,2],        [1,2])   ==> [ [null,null], [null,null] ]
//        diffObjs([1,2],        [2,1])   ==> [ [1,2], [2,1] ]
//        diffObjs([1,2],        [1,2,3]) ==> [ [1,2,null], [2,1,3] ]
//        diffObjs([1,1,2,3],    [1,2,3]) ==> [ [null,1,2,3], [null,2,3] ]
//

var diffObjs = function(v1, v2) {

    // return NULL when passed references to
    // the same objects or matching scalar values
    if (v1 === v2) {
        return null;
    }
    var cloneIt = function(v) {
        if (v == null || typeof v != 'object') {
            return v;
        }

        var isArray = Array.isArray(v);

        var obj = isArray ? [] : {};
        if (!isArray) {
            // handles function, etc
            Object.assign({}, v);
        }

        for (var i in v) {
            obj[i] = cloneIt(v[i]);
        }

        return obj;
    }

    // different types or array compared to non-array
    if (typeof v1 != typeof v2 || Array.isArray(v1) != Array.isArray(v2)) {
        return [cloneIt(v1), cloneIt(v2)];
    }

    // different scalars (no cloning needed)
    if (typeof v1 != 'object' && v1 !== v2) {
        return [v1, v2];
    }

    // one is null, the other isn't
    // (if they were both null, the '===' comparison
    // above would not have allowed us here)
    if (v1 == null || v2 == null) {
        return [cloneIt(v1), cloneIt(v2)]; 
    }

    // We have two objects or two arrays to compare.
    var isArray = Array.isArray(v1);

    var left = isArray ? [] : {};
    var right = isArray ? [] : {};

    for (var i in v1) {
        if (!v2.hasOwnProperty(i)) {
            left[i] = cloneIt(v1[i]);
        } else {
            var sub_diff = diffObjs(v1[i], v2[i]);
            // copy the differences between the 
            // two objects into the results.
            // - If the object is array, use 'null'
            //   to indicate the two corresponding elements
            //   match.
            //
            // - If the object is not an array, copy only
            //   the members that point to an unmatched
            //   object.
            if (isArray || sub_diff) { 
                left[i] = sub_diff ? cloneIt(sub_diff[0]) : null;
                right[i] = sub_diff ? cloneIt(sub_diff[1]) : null;
            }
        }
    }

    for (var i in v2) {
        if (!v1.hasOwnProperty(i)) {
            right[i] = cloneIt(v2[i]);
        }
    }

    return [ left, right];
};