JavaScript implementation for constructing objects diffs compatible with JSON PATCH syntax.

jsondiff.js

var jsondiff = (function() {

    // Patch helper functions
    function getParent(paths, path) {
        return paths[path.substr(0, path.match(/\//g).length)];
    }

    // Checks if `obj` is an array or object
    function isContainer(obj) {
        return _.isArray(obj) || _.isObject(obj);
    }

    // Checks if the two objects are of the same container type
    function isSameContainer(obj1, obj2) {
        return (_.isArray(obj1) && _.isArray(obj2)) || (_.isObject(obj1) && _.isObject(obj2));
    }

    // Flattens an object to a hash of paths and values.
    function flattenObject(obj, prefix, paths) {
        prefix || (prefix = '/');
        paths || (paths = {});

        // Do not bother logging the root path
        paths[prefix] = {
            path: prefix,
            value: obj
        };

        prefix !== '/' && (prefix = prefix + '/')

        // Recurse for container types
        if (_.isArray(obj)) {
            for (var i = 0, l = obj.length; i < l; i++) {
                flattenObject(obj[i], prefix + i, paths);
            }
        } else if (_.isObject(obj)) {
            for (var key in obj) {
                flattenObject(obj[key], prefix + key, paths);
            }
        }

        return paths;
    }

    // Constructs a patch that when applied to `obj2`, it will be equivalent
    // to `obj1`. The patch format conforms to IETF JSON Patch proposal
    // http://tools.ietf.org/html/draft-ietf-appsawg-json-patch-01
    function constructPatch(obj1, obj2) {
        // Patches are only applicable to two of the same container types.
        if (!isSameContainer(obj1, obj2)) {
            throw new Error('Patches can only be derived from objects or arrays');
        }

        var paths1 = flattenObject(obj1),
            paths2 = flattenObject(obj2),
            key1,
            key2,
            doc1,
            doc2,
            patch = [],
            add = {},
            remove = {},
            replace = {},
            move = {};

        // Iterate over the first object's paths and compare them to the second
        // set of paths.
        for (key1 in paths1) {
            doc1 = paths1[key1], doc2 = paths2[key1];
            // If the parent of `doc2` doesn't exist, skip it since neither a
            // remove or replace can occur.
            if (!getParent(paths2, key1)) {
                continue;
            }
            // If there is a miss in the second object, the key will be marked for
            // removal.
            if (!doc2) {
                remove[key1] = doc1;
            // If both members have existing values, make sure they are not the
            // same container and they are not equal. If they are the same
            // container type, values will be replaced downstream.
            } else if (!isSameContainer(doc1.value, doc2.value) && !_.isEqual(doc1.value, doc2.value)) {
                replace[key1] = doc2;
            }
        }

        // Iterate over the second object's paths and compare them to the first
        // set of paths.
        for (key2 in paths2) {
            doc1 = paths1[key2], doc2 = paths2[key2];
            // Missing in first object, thus we mark it to be added.
            // If the parent path is not present in the first obj, then this
            // means the whole array/object is new.
            if (!doc1 && isSameContainer(getParent(paths1, key2), getParent(paths2, key2))) {
                add[key2] = doc2;
            }
        }

        // Attempt to promote add/remove operations to a move operation.
        // The first occurence of the same value, we can promote to a move.
        for (key1 in remove) {
            doc1 = remove[key1];
            for (key2 in add) {
                doc2 = add[key2];
                if (_.isEqual(doc2.value, doc1.value)) {
                    // Remove them from previous hashes
                    delete remove[key1];
                    delete add[key2];
                    move[key1] = doc2
                    break;
                }
            }
        }

        var key;
        // Populate the patch
        for (key in add) {
            patch.push({add: key, value: add[key].value});
        }
        for (key in remove) {
            patch.push({remove: key});
        }
        for (key in replace) {
            patch.push({replace: key, value: replace[key].value});
        }
        for (key in move) {
            patch.push({move: key, to: move[key].path, value: move[key].value});
        }

        return patch;
    }

    return constructPatch;

})();

Function getParent() is broken. Use

    function getParent(paths, path) {
        return paths[path.replace(/\/[^\/]*$/, '')];
    }

Bug: Keys must not contain a slash!