Question

TL;DR version: I want to avoid adding duplicate Javascript objects to an array of similar objects, some of which might be really big. What’s the best approach?

I have an application where I’m loading large amounts of JSON data into a Javascript data structure. While it’s a bit more complex than this, assume that I’m loading JSON into an array of Javascript objects from a server through a series of AJAX requests, something like:

var myObjects = [];

function processObject(o) {
    myObjects.push(o);
}

for (var x=0; x<1000; x++) {
    $.getJSON('/new_object.json', processObject);
}

To complicate matters, the JSON:

• is in an unknown schema
• is of arbitrary length (probably not enormous, but could be in the 100-200 kb range)
• might contain duplicates across different requests

My initial thought is to have an additional object to store a hash of each object (via JSON.stringify?) and check against it on each load, like this:

var myHashMap = {};

function processObject(o) {
    var hash = JSON.stringify(o);
    // is it in the hashmap?
    if (!(myHashMap[hash])) {
        myObjects.push(o);
        // set the hashmap key for future checks
        myHashMap[hash] = true;
    }
    // else ignore this object
}

but I’m worried about having property names in myHashMap that might be 200 kb in length. So my questions are:

• Is there a better approach for this problem than the hashmap idea?
• If not, is there a better way to make a hash function for a JSON object of arbitrary length and schema than JSON.stringify?
• What are the possible issues with super-long property names in an object?

Answer 1

I’d suggest you create an MD5 hash of the JSON.stringify(o) and store that in your hashmap with a reference to your stored object as the data for the hash. And to make sure that there are no object key order differences in the JSON.stringify(), you have to create a copy of the object that orders the keys.

Then, when each new object comes in, you check it against the hash map. If you find a match in the hash map, then you compare the incoming object with the actual object that you’ve stored to see if they are truly duplicates (since there can be MD5 hash collisions). That way, you have a manageable hash table (with only MD5 hashes in it).

Here’s code to create a canonical string representation of an object (including nested objects or objects within arrays) that handles object keys that might be in a different order if you just called JSON.stringify().

// Code to do a canonical JSON.stringify() that puts object properties 
// in a consistent order
// Does not allow circular references (child containing reference to parent)
JSON.stringifyCanonical = function(obj) {
    // compatible with either browser or node.js
    var Set = typeof window === "object" ? window.Set : global.Set;

    // poor man's Set polyfill
    if (typeof Set !== "function") {
        Set = function(s) {
            if (s) {
                this.data = s.data.slice();
            } else {
                this.data = [];
            }
        };
        Set.prototype = {
            add: function(item) {
                this.data.push(item);
            },
            has: function(item) {
                return this.data.indexOf(item) !== -1;
            }
        };
    }

    function orderKeys(obj, parents) {
        if (typeof obj !== "object") {
            throw new Error("orderKeys() expects object type");
        }
        var set = new Set(parents);
        if (set.has(obj)) {
            throw new Error("circular object in stringifyCanonical()");
        }
        set.add(obj);
        var tempObj, item, i;
        if (Array.isArray(obj)) {
            // no need to re-order an array
            // but need to check it for embedded objects that need to be ordered
            tempObj = [];
            for (i = 0; i < obj.length; i++) {
                item = obj[i];
                if (typeof item === "object") {
                    tempObj[i] = orderKeys(item, set);
                } else {
                    tempObj[i] = item;
                }
            }
        } else {
            tempObj = {};
            // get keys, sort them and build new object
            Object.keys(obj).sort().forEach(function(item) {
                if (typeof obj[item] === "object") {
                    tempObj[item] = orderKeys(obj[item], set);
                } else {
                    tempObj[item] = obj[item];
                }
            });
        }
        return tempObj;
    }

    return JSON.stringify(orderKeys(obj));
}

And, the algorithm

var myHashMap = {};

function processObject(o) {
    var stringifiedCandidate = JSON.stringifyCanonical(o);
    var hash = CreateMD5(stringifiedCandidate);
    var list = [], found = false;
    // is it in the hashmap?
    if (!myHashMap[hash] {
        // not in the hash table, so it's a unique object
        myObjects.push(o);
        list.push(myObjects.length - 1);    // put a reference to the object with this hash value in the list
        myHashMap[hash] = list;             // store the list in the hash table for future comparisons
    } else {
        // the hash does exist in the hash table, check for an exact object match to see if it's really a duplicate
        list = myHashMap[hash];             // get the list of other object indexes with this hash value
        // loop through the list
        for (var i = 0; i < list.length; i++) {
            if (stringifiedCandidate === JSON.stringifyCanonical(myObjects[list[i]])) {
                found = true;       // found an exact object match
                break;
            }
        }
        // if not found, it's not an exact duplicate, even though there was a hash match
        if (!found) {
            myObjects.push(o);
            myHashMap[hash].push(myObjects.length - 1);
        }
    }
}

Test case for jsonStringifyCanonical() is here: https://jsfiddle.net/jfriend00/zfrtpqcL/