Is there hash code function accepting any object type?

Question

Basically, I'm trying to create an object of unique objects, a set. I had the brilliant idea of just using a JavaScript object with objects for the property names. Such as,

set[obj] = true;

This works, up to a point. It works great with string and numbers, but with other objects, they all seem to "hash" to the same value and access the same property. Is there some kind of way I can generate a unique hash value for an object? How do strings and numbers do it, can I override the same behavior?

Answer 1

If you want a hashCode() function like Java's in JavaScript, that is yours:

function hashCode(string){
    var hash = 0;
    for (var i = 0; i < string.length; i++) {
        var code = string.charCodeAt(i);
        hash = ((hash<<5)-hash)+code;
        hash = hash & hash; // Convert to 32bit integer
    }
    return hash;
}

That is the way of implementation in Java (bitwise operator).

Please note that hashCode could be positive and negative, and that's normal, see HashCode giving negative values. So, you could consider to use Math.abs() along with this function.

Answer 2

JavaScript objects can only use strings as keys (anything else is converted to a string).

You could, alternatively, maintain an array which indexes the objects in question, and use its index string as a reference to the object. Something like this:

var ObjectReference = [];
ObjectReference.push(obj);

set['ObjectReference.' + ObjectReference.indexOf(obj)] = true;

Obviously it's a little verbose, but you could write a couple of methods that handle it and get and set all willy nilly.

Edit:

Your guess is fact -- this is defined behaviour in JavaScript -- specifically a toString conversion occurs meaning that you can can define your own toString function on the object that will be used as the property name. - olliej

This brings up another interesting point; you can define a toString method on the objects you want to hash, and that can form their hash identifier.

Answer 3

The easiest way to do this is to give each of your objects its own unique toString method:

(function() {
    var id = 0;

    /*global MyObject */
    MyObject = function() {
        this.objectId = '<#MyObject:' + (id++) + '>';
        this.toString= function() {
            return this.objectId;
        };
    };
})();

I had the same problem and this solved it perfectly for me with minimal fuss, and was a lot easier that re-implementing some fatty Java style Hashtable and adding equals() and hashCode() to your object classes. Just make sure that you don't also stick a string '<#MyObject:12> into your hash or it will wipe out the entry for your exiting object with that id.

Now all my hashes are totally chill. I also just posted a blog entry a few days ago about this exact topic.

Answer 4

What you described is covered by Harmony WeakMaps, part of the ECMAScript 6 specification (next version of JavaScript). That is: a set where the keys can be anything (including undefined) and is non-enumerable.

This means it's impossible to get a reference to a value unless you have a direct reference to the key (any object!) that links to it. It's important for a bunch of engine implementation reasons relating to efficiency and garbage collection, but it's also super cool for in that it allows for new semantics like revokable access permissions and passing data without exposing the data sender.

From MDN:

var wm1 = new WeakMap(),
    wm2 = new WeakMap();
var o1 = {},
    o2 = function(){},
    o3 = window;

wm1.set(o1, 37);
wm1.set(o2, "azerty");
wm2.set(o1, o2); // A value can be anything, including an object or a function.
wm2.set(o3, undefined);
wm2.set(wm1, wm2); // Keys and values can be any objects. Even WeakMaps!

wm1.get(o2); // "azerty"
wm2.get(o2); // Undefined, because there is no value for o2 on wm2.
wm2.get(o3); // Undefined, because that is the set value.

wm1.has(o2); // True
wm2.has(o2); // False
wm2.has(o3); // True (even if the value itself is 'undefined').

wm1.has(o1);   // True
wm1.delete(o1);
wm1.has(o1);   // False

WeakMaps are available in current Firefox, Chrome and Edge. They're also supported in Node v7 , and in v6 with the --harmony-weak-maps flag.

Answer 5

The solution I chose is similar to Daniel's, but rather than use an object factory and override the toString, I explicitly add the hash to the object when it is first requested through a getHashCode function. A little messy, but better for my needs :)

Function.prototype.getHashCode = (function(id) {
    return function() {
        if (!this.hashCode) {
            this.hashCode = '<hash|#' + (id++) + '>';
        }
        return this.hashCode;
    }
}(0));

Answer 6

For my specific situation I only care about the equality of the object as far as keys and primitive values go. The solution that worked for me was converting the object to its JSON representation and using that as the hash. There are limitations such as order of key definition potentially being inconsistent; but like I said it worked for me because these objects were all being generated in one place.

var hashtable = {};

var myObject = {a:0,b:1,c:2};

var hash = JSON.stringify(myObject);
// '{"a":0,"b":1,"c":2}'

hashtable[hash] = myObject;
// {
//   '{"a":0,"b":1,"c":2}': myObject
// }

Answer 7

I put together a small JavaScript module a while ago to produce hashcodes for strings, objects, arrays, etc. (I just committed it to GitHub :) )

Usage:

Hashcode.value("stackoverflow")
// -2559914341
Hashcode.value({ 'site' : "stackoverflow" })
// -3579752159

Answer 8

In ECMAScript 6 there's now a Set that works how you'd like: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set

It's already available in the latest Chrome, FF, and IE11.

Answer 9

The JavaScript specification defines indexed property access as performing a toString conversion on the index name. For example,

myObject[myProperty] = ...;

is the same as

myObject[myProperty.toString()] = ...;

This is necessary as in JavaScript

myObject["someProperty"]

is the same as

myObject.someProperty

And yes, it makes me sad as well :-(

Answer 10

Based on the title, we can generate strong SHA hashes, in a browser context, it can be used to generate a unique hash from an object, an array of params, a string, or whatever.

async function H(m) {
  const msgUint8 = new TextEncoder().encode(m)                       
  const hashBuffer = await crypto.subtle.digest('SHA-256', msgUint8)          
  const hashArray = Array.from(new Uint8Array(hashBuffer))                    
  const hashHex = hashArray.map(b => b.toString(16).padStart(2, '0')).join('')
  console.log(hashHex)
}

/* Examples ----------------------- */
H("An obscure ....")
H(JSON.stringify( {"hello" : "world"} ))
H(JSON.stringify( [54,51,54,47] ))

The above output in my browser, it should be equal for you too:

bf1cf3fe6975fe382ab392ec1dd42009380614be03d489f23601c11413cfca2b
93a23971a914e5eacbf0a8d25154cda309c3c1c72fbb9914d47c60f3cb681588
d2f209e194045604a3b15bdfd7502898a0e848e4603c5a818bd01da69c00ad19

Supported algos:

SHA-1 (but don't use this in cryptographic applications)
SHA-256
SHA-384
SHA-512

https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto/digest#Converting_a_digest_to_a_hex_string

---

However, for a simple FAST checksum hash function, made only for collision avoidance, see CRC32 (Content Redundancy Check)

JavaScript CRC32

---

You might also be interested by this similar method to generate HMAC codes via the web crypto api.

Answer 11

Reference: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol

you can use Es6 symbol to create unique key and access object. Every symbol value returned from Symbol() is unique. A symbol value may be used as an identifier for object properties; this is the data type's only purpose.

var obj = {};

obj[Symbol('a')] = 'a';
obj[Symbol.for('b')] = 'b';
obj['c'] = 'c';
obj.d = 'd';

Answer 12

Here's my simple solution that returns a unique integer.

function hashcode(obj) {
    var hc = 0;
    var chars = JSON.stringify(obj).replace(/\{|\"|\}|\:|,/g, '');
    var len = chars.length;
    for (var i = 0; i < len; i++) {
        // Bump 7 to larger prime number to increase uniqueness
        hc += (chars.charCodeAt(i) * 7);
    }
    return hc;
}

Answer 13

My solution introduces a static function for the global Object object.

(function() {
    var lastStorageId = 0;

    this.Object.hash = function(object) {
        var hash = object.__id;

        if (!hash)
             hash = object.__id = lastStorageId++;

        return '#' + hash;
    };
}());

I think this is more convenient with other object manipulating functions in JavaScript.

Answer 14

I will try to go a little deeper than other answers.

Even if JS had better hashing support it would not magically hash everything perfectly, in many cases you will have to define your own hash function. For example Java has good hashing support, but you still have to think and do some work.

One problem is with the term hash/hashcode ... there is cryptographic hashing and non-cryptographic hashing. The other problem, is you have to understand why hashing is useful and how it works.

When we talk about hashing in JavaScript or Java most of the time we are talking about non-cryptographic hashing, usually about hashing for hashmap/hashtable (unless we are working on authentication or passwords, which you could be doing server-side using NodeJS ...).

It depends on what data you have and what you want to achieve.

Your data has some natural "simple" uniqueness:

• The hash of an integer is ... the integer, as it is unique, lucky you !
• The hash of a string ... it depends on the string, if the string represents a unique identifier, you may consider it as a hash (so no hashing needed).
• Anything which is indirectly pretty much a unique integer is the simplest case
• This will respect: hashcode equal if objects are equal

Your data has some natural "composite" uniqueness:

• For example with a person object, you may compute a hash using firstname, lastname, birthdate, ... see how Java does it: Good Hash Function for Strings, or use some other ID info that is cheap and unique enough for your usecase

You have no idea what your data will be:

• Good luck ... you could serialize to string and hash it Java style, but that may be expensive if the string is large and it will not avoid collisions as well as say the hash of an integer (self).

There is no magically efficient hashing technique for unknown data, in some cases it is quite easy, in other cases you may have to think twice. So even if JavaScript/ECMAScript adds more support, there is no magic language solution for this problem.

In practice you need two things: enough uniqueness, enough speed

In addition to that it is great to have: "hashcode equal if objects are equal"

• https://en.wikipedia.org/wiki/Hash_table#Collision_resolution
• Relationship between hashCode and equals method in Java

Answer 15

I combined the answers from eyelidlessness and KimKha.

The following is an angularjs service and it supports numbers, strings, and objects.

exports.Hash = () => {
  let hashFunc;
  function stringHash(string, noType) {
    let hashString = string;
    if (!noType) {
      hashString = `string${string}`;
    }
    var hash = 0;
    for (var i = 0; i < hashString.length; i++) {
        var character = hashString.charCodeAt(i);
        hash = ((hash<<5)-hash)+character;
        hash = hash & hash; // Convert to 32bit integer
    }
    return hash;
  }

  function objectHash(obj, exclude) {
    if (exclude.indexOf(obj) > -1) {
      return undefined;
    }
    let hash = '';
    const keys = Object.keys(obj).sort();
    for (let index = 0; index < keys.length; index += 1) {
      const key = keys[index];
      const keyHash = hashFunc(key);
      const attrHash = hashFunc(obj[key], exclude);
      exclude.push(obj[key]);
      hash += stringHash(`object${keyHash}${attrHash}`, true);
    }
    return stringHash(hash, true);
  }

  function Hash(unkType, exclude) {
    let ex = exclude;
    if (ex === undefined) {
      ex = [];
    }
    if (!isNaN(unkType) && typeof unkType !== 'string') {
      return unkType;
    }
    switch (typeof unkType) {
      case 'object':
        return objectHash(unkType, ex);
      default:
        return stringHash(String(unkType));
    }
  }

  hashFunc = Hash;

  return Hash;
};

Example Usage:

Hash('hello world'), Hash('hello world') == Hash('hello world')
Hash({hello: 'hello world'}), Hash({hello: 'hello world'}) == Hash({hello: 'hello world'})
Hash({hello: 'hello world', goodbye: 'adios amigos'}), Hash({hello: 'hello world', goodbye: 'adios amigos'}) == Hash({goodbye: 'adios amigos', hello: 'hello world'})
Hash(['hello world']), Hash(['hello world']) == Hash(['hello world'])
Hash(1), Hash(1) == Hash(1)
Hash('1'), Hash('1') == Hash('1')

Output

432700947 true
-411117486 true
1725787021 true
-1585332251 true
1 true
-1881759168 true

Explanation

As you can see the heart of the service is the hash function created by KimKha.I have added types to the strings so that the sturucture of the object would also impact the final hash value.The keys are hashed to prevent array|object collisions.

eyelidlessness object comparision is used to prevent infinit recursion by self referencing objects.

Usage

I created this service so that I could have an error service that is accessed with objects. So that one service can register an error with a given object and another can determine if any errors were found.

ie

JsonValidation.js

ErrorSvc({id: 1, json: '{attr: "not-valid"}'}, 'Invalid Json Syntax - key not double quoted');

UserOfData.js

ErrorSvc({id: 1, json: '{attr: "not-valid"}'});

This would return:

['Invalid Json Syntax - key not double quoted']

While

ErrorSvc({id: 1, json: '{"attr": "not-valid"}'});

This would return

[]

Answer 16

In addition to eyelidlessness's answer, here is a function that returns a reproducible, unique ID for any object:

var uniqueIdList = [];
function getConstantUniqueIdFor(element) {
    // HACK, using a list results in O(n), but how do we hash e.g. a DOM node?
    if (uniqueIdList.indexOf(element) < 0) {
        uniqueIdList.push(element);
    }
    return uniqueIdList.indexOf(element);
}

As you can see it uses a list for look-up which is very inefficient, however that's the best I could find for now.

Answer 17

If you want to use objects as keys you need to overwrite their toString Method, as some already mentioned here. The hash functions that were used are all fine, but they only work for the same objects not for equal objects.

I've written a small library that creates hashes from objects, which you can easily use for this purpose. The objects can even have a different order, the hashes will be the same. Internally you can use different types for your hash (djb2, md5, sha1, sha256, sha512, ripemd160).

Here is a small example from the documentation:

var hash = require('es-hash');

// Save data in an object with an object as a key
Object.prototype.toString = function () {
    return '[object Object #'+hash(this)+']';
}

var foo = {};

foo[{bar: 'foo'}] = 'foo';

/*
 * Output:
 *  foo
 *  undefined
 */
console.log(foo[{bar: 'foo'}]);
console.log(foo[{}]);

The package can be used either in browser and in Node-Js.

Repository: https://bitbucket.org/tehrengruber/es-js-hash

Answer 18

If you truly want set behavior (I'm going by Java knowledge), then you will be hard pressed to find a solution in JavaScript. Most developers will recommend a unique key to represent each object, but this is unlike set, in that you can get two identical objects each with a unique key. The Java API does the work of checking for duplicate values by comparing hash code values, not keys, and since there is no hash code value representation of objects in JavaScript, it becomes almost impossible to do the same. Even the Prototype JS library admits this shortcoming, when it says:

"Hash can be thought of as an associative array, binding unique keys to values (which are not necessarily unique)..."

http://www.prototypejs.org/api/hash

Answer 19

If you want to have unique values in a lookup object you can do something like this:

Creating a lookup object

var lookup = {};

Setting up the hashcode function

function getHashCode(obj) {
    var hashCode = '';
    if (typeof obj !== 'object')
        return hashCode + obj;
    for (var prop in obj) // No hasOwnProperty needed
        hashCode += prop + getHashCode(obj[prop]); // Add key + value to the result string
    return hashCode;
}

Object

var key = getHashCode({ 1: 3, 3: 7 });
// key = '1337'
lookup[key] = true;

Array

var key = getHashCode([1, 3, 3, 7]);
// key = '01132337'
lookup[key] = true;

Other types

var key = getHashCode('StackOverflow');
// key = 'StackOverflow'
lookup[key] = true;

Final result

{ 1337: true, 01132337: true, StackOverflow: true }

Do note that getHashCode doesn't return any value when the object or array is empty

getHashCode([{},{},{}]);
// '012'
getHashCode([[],[],[]]);
// '012'

This is similar to @ijmacd solution only getHashCode doesn't has the JSON dependency.

Answer 20

Just use hidden secret property with the defineProperty enumerable: false

It work very fast:

• The first read uniqueId: 1,257,500 ops/s
• All others: 309,226,485 ops/s

var nextObjectId = 1
function getNextObjectId() {
    return nextObjectId++
}

var UNIQUE_ID_PROPERTY_NAME = '458d576952bc489ab45e98ac7f296fd9'
function getObjectUniqueId(object) {
    if (object == null) {
        return null
    }

    var id = object[UNIQUE_ID_PROPERTY_NAME]

    if (id != null) {
        return id
    }

    if (Object.isFrozen(object)) {
        return null
    }

    var uniqueId = getNextObjectId()
    Object.defineProperty(object, UNIQUE_ID_PROPERTY_NAME, {
        enumerable: false,
        configurable: false,
        writable: false,
        value: uniqueId,
    })

    return uniqueId
}