[javascript] Map vs Object in JavaScript

I just discovered this feature:

Map: Map objects are simple key/value maps.

That confused me. Regular JavaScript objects are dictionaries, so how is a Map different from a dictionary? Conceptually, they're identical (according to another question on Stackoverflow)

The documentation doesn't help either:

Map objects are collections of key/value pairs where both the keys and values may be arbitrary ECMAScript language values. A distinct key value may only occur in one key/value pair within the Map’s collection. Distinct key values as discriminated using the a comparision algorithm that is selected when the Map is created.

A Map object can iterate its elements in insertion order. Map object must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structures used in this Map objects specification is only intended to describe the required observable semantics of Map objects. It is not intended to be a viable implementation model.

…still sounds like an object to me, so clearly I've missed something.

Why is JavaScript gaining a (well-supported) Map object? What does it do?

I came across this post by Minko Gechev which clearly explains the major differences.

The key difference is that Objects only support string and Symbol keys where as Maps support more or less any key type.

If I do obj[123] = true and then Object.keys(obj) then I will get ["123"] rather than [123]. A Map would preserve the type of the key and return [123] which is great. Maps also allow you to use Objects as keys. Traditionally to do this you would have to give objects some kind of unique identifier to hash them (I don't think I've ever seen anything like getObjectId in JS as part of the standard). Maps also guarantee preservation of order so are all round better for preservation and can sometimes save you needing to do a few sorts.

Between maps and objects in practice there are several pros and cons. Objects gain both advantages and disadvantages being very tightly integrated into the core of JavaScript which sets them apart from significantly Map beyond the difference in key support.

An immediate advantage is that you have syntactical support for Objects making it easy to access elements. You also have direct support for it with JSON. When used as a hash it's annoying to get an object without any properties at all. By default if you want to use Objects as a hash table they will be polluted and you will often have to call hasOwnProperty on them when accessing properties. You can see here how by default Objects are polluted and how to create hopefully unpolluted objects for use as hashes:

({}).toString
    toString() { [native code] }
JSON.parse('{}').toString
    toString() { [native code] }
(Object.create(null)).toString
    undefined
JSON.parse('{}', (k,v) => (typeof v === 'object' && Object.setPrototypeOf(v, null) ,v)).toString
    undefined

Pollution on objects is not only something that makes code more annoying, slower, etc but can also have potential consequences for security.

Objects are not pure hash tables but are trying to do more. You have headaches like hasOwnProperty, not being able to get the length easily (Object.keys(obj).length) and so on. Objects are not meant to purely be used as hash maps but as dynamic extensible Objects as well and so when you use them as pure hash tables problems arise.

Comparison/List of various common operations:

    Object:
       var o = {};
       var o = Object.create(null);
       o.key = 1;
       o.key += 10;
       for(let k in o) o[k]++;
       var sum = 0;
       for(let v of Object.values(m)) sum += v;
       if('key' in o);
       if(o.hasOwnProperty('key'));
       delete(o.key);
       Object.keys(o).length
    Map:
       var m = new Map();
       m.set('key', 1);
       m.set('key', m.get('key') + 10);
       m.foreach((k, v) => m.set(k, m.get(k) + 1));
       for(let k of m.keys()) m.set(k, m.get(k) + 1);
       var sum = 0;
       for(let v of m.values()) sum += v;
       if(m.has('key'));
       m.delete('key');
       m.size();

There are a few other options, approaches, methodologies, etc with varying ups and downs (performance, terse, portable, extendable, etc). Objects are a bit strange being core to the language so you have a lot of static methods for working with them.

Besides the advantage of Maps preserving key types as well as being able to support things like objects as keys they are isolated from the side effects that objects much have. A Map is a pure hash, there's no confusion about trying to be an object at the same time. Maps can also be easily extended with proxy functions. Object's currently have a Proxy class however performance and memory usage is grim, in fact creating your own proxy that looks like Map for Objects currently performs better than Proxy.

A substantial disadvantage for Maps is that they are not supported with JSON directly. Parsing is possible but has several hangups:

JSON.parse(str, (k,v) => {
    if(typeof v !== 'object') return v;
    let m = new Map();
    for(k in v) m.set(k, v[k]);
    return m;
});

The above will introduce a serious performance hit and will also not support any string keys. JSON encoding is even more difficult and problematic (this is one of many approaches):

// An alternative to this it to use a replacer in JSON.stringify.
Map.prototype.toJSON = function() {
    return JSON.stringify({
        keys: Array.from(this.keys()),
        values: Array.from(this.values())
    });
};

This is not so bad if you're purely using Maps but will have problems when you are mixing types or using non-scalar values as keys (not that JSON is perfect with that kind of issue as it is, IE circular object reference). I haven't tested it but chances are that it will severely hurt performance compared to stringify.

Other scripting languages often don't have such problems as they have explicit non-scalar types for Map, Object and Array. Web development is often a pain with non-scalar types where you have to deal with things like PHP merges Array/Map with Object using A/M for properties and JS merges Map/Object with Array extending M/O. Merging complex types is the devil's bane of high level scripting languages.

So far these are largely issues around implementation but performance for basic operations is important as well. Performance is also complex because it depends on engine and usage. Take my tests with a grain of salt as I cannot rule out any mistake (I have to rush this). You should also run your own tests to confirm as mine examine only very specific simple scenarios to give a rough indication only. According to tests in Chrome for very large objects/maps the performance for objects is worse because of delete which is apparently somehow proportionate to the number of keys rather than O(1):

Object Set Took: 146
Object Update Took: 7
Object Get Took: 4
Object Delete Took: 8239
Map Set Took: 80
Map Update Took: 51
Map Get Took: 40
Map Delete Took: 2

Chrome clearly has a strong advantage with getting and updating but the delete performance is horrific. Maps use a tiny amount more memory in this case (overhead) but with only one Object/Map being tested with millions of keys the impact of overhead for maps is not expressed well. With memory management objects also do seem to free earlier if I am reading the profile correctly which might be one benefit in favor of objects.

In Firefox for this particular benchmark it is a different story:

Object Set Took: 435
Object Update Took: 126
Object Get Took: 50
Object Delete Took: 2
Map Set Took: 63
Map Update Took: 59
Map Get Took: 33
Map Delete Took: 1

I should immediately point out that in this particular benchmark deleting from objects in Firefox is not causing any problems, however in other benchmarks it has caused problems especially when there are many keys just as in Chrome. Maps are clearly superior in Firefox for large collections.

However this is not the end of the story, what about many small objects or maps? I have done a quick benchmark of this but not an exhaustive one (setting/getting) of which performs best with a small number of keys in the above operations. This test is more about memory and initialization.

Map Create: 69    // new Map
Object Create: 34 // {}

Again these figures vary but basically Object has a good lead. In some cases the lead for Objects over maps is extreme (~10 times better) but on average it was around 2-3 times better. It seems extreme performance spikes can work both ways. I only tested this in Chrome and creation to profile memory usage and overhead. I was quite surprised to see that in Chrome it appears that Maps with one key use around 30 times more memory than Objects with one key.

For testing many small objects with all the above operations (4 keys):

Chrome Object Took: 61
Chrome Map Took: 67
Firefox Object Took: 54
Firefox Map Took: 139

In terms of memory allocation these behaved the same in terms of freeing/GC but Map used 5 times more memory. This test used 4 keys where as in the last test I only set one key so this would explain the reduction in memory overhead. I ran this test a few times and Map/Object are more or less neck and neck overall for Chrome in terms of overall speed. In Firefox for small Objects there is a definite performance advantage over maps overall.

This of course doesn't include the individual options which could vary wildly. I would not advice micro-optimizing with these figures. What you can get out of this is that as a rule of thumb, consider Maps more strongly for very large key value stores and objects for small key value stores.

Beyond that the best strategy with these two it to implement it and just make it work first. When profiling it is important to keep in mind that sometimes things that you wouldn't think would be slow when looking at them can be incredibly slow because of engine quirks as seen with the object key deletion case.

These two tips can help you to decide whether to use a Map or an Object:

• Use maps over objects when keys are unknown until run time, and when all keys are the same type and all values are the same type.

• Use maps in case if there is a need to store primitive values as keys because object treats each key as a string either its a number value, boolean value or any other primitive value.

• Use objects when there is logic that operates on individual elements.

Source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Keyed_Collections#Object_and_Map_compared

One aspect of the Map that is not given much press here is lookup. According to the spec:

A Map object must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structures used in this Map objects specification is only intended to describe the required observable semantics of Map objects. It is not intended to be a viable implementation model.

For collections that have a huge number of items and require item lookups, this is a huge performance boost.

TL;DR - Object lookup is not specified, so it can be on order of the number of elements in the object, i.e., O(n). Map lookup must use a hash table or similar, so Map lookup is the same regardless of Map size, i.e. O(1).

When to use Maps instead of plain JavaScript Objects ?

The plain JavaScript Object { key: 'value' } holds structured data. But plain JS object has its limitations :

1. Only strings and symbols can be used as keys of Objects. If we use any other things say, numbers as keys of an object then during accessing those keys we will see those keys will be converted into strings implicitly causing us to lose consistency of types. const names= {1: 'one', 2: 'two'}; Object.keys(names); // ['1', '2']

2. There are chances of accidentally overwriting inherited properties from prototypes by writing JS identifiers as key names of an object (e.g. toString, constructor etc.)

3. Another object cannot be used as key of an object, so no extra information can be written for an object by writing that object as key of another object and value of that another object will contain the extra information

4. Objects are not iterators

5. The size of an object cannot be determined directly

These limitations of Objects are solved by Maps but we must consider Maps as complement for Objects instead of replacement. Basically Map is just array of arrays but we must pass that array of arrays to the Map object as argument with new keyword otherwise only for array of arrays the useful properties and methods of Map aren't available. And remember key-value pairs inside the array of arrays or the Map must be separated by commas only, no colons like in plain objects.

3 tips to decide whether to use a Map or an Object :

1. Use maps over objects when keys are unknown until run time because keys formed by user input or unknowingly can break the code which uses the object if those keys overwrite the inherited properties of the object, so map is safer in those cases. Also use maps when all keys are the same type and all maps are the same type.

2. Use maps if there is a need to store primitive values as keys.

3. Use objects if we need to operate on individual elements.

Benefits of using Maps are :

1. Map accepts any key type and preserves the type of key :

We know that if the object's key is not a string or symbol then JS implicitly transforms it into a string. On the contrary, Map accepts any type of keys : string, number, boolean, symbol etc. and Map preserves the original key type. Here we will use number as key inside a Map and it will remain a number :

const numbersMap= new Map();

numbersMap.set(1, 'one');

numbersMap.set(2, 'two');

const keysOfMap= [...numbersMap.keys()];

console.log(keysOfMap);                        // [1, 2]

Inside a Map we can even use an entire object as a key. There may be times when we want to store some object related data, without attaching this data inside the object itself so that we can work with lean objects but want to store some information about the object. In those cases we need to use Map so that we can make Object as key and related data of the object as value.

const foo= {name: foo};

const bar= {name: bar};

const kindOfMap= [[foo, 'Foo related data'], [bar, 'Bar related data']];

But the downside of this approach is the complexity of accessing the value by key, as we have to loop through the entire array to get the desired value.

function getBy Key(kindOfMap, key) {
    for (const [k, v]  of kindOfMap) {
        if(key === k) {
            return v;
        }
    }
    return undefined;
}

getByKey(kindOfMap, foo);            // 'Foo related data'

We can solve this problem of not getting direct access to the value by using a proper Map.

const foo= {name: 'foo'};

const bar= {name: 'bar'};

const myMap= new Map();

myMap.set(foo, 'Foo related data');
myMap.set(bar, 'Bar related data');

console.log(myMap.get(foo));            // 'Foo related data'

We could have done this using WeakMap, just have to write, const myMap= new WeakMap( ). The differences between Map and WeakMap are that WeakMap allows for garbage collection of keys (here objects) so it prevents memory leaks, WeakMap accepts only objects as keys, and WeakMap has reduced set of methods.

2. Map has no restriction over key names :

For plain JS objects we can accidentally overwrite property inherited from the prototype and it can be dangerous. Here we will overwrite the toString( ) property of the actor object :

const actor= {
    name: 'Harrison Ford',
    toString: 'Actor: Harrison Ford'
};

Now let's define a fn isPlainObject( ) to determine if the supplied argument is a plain object and this fn uses toString( ) method to check it :

function isPlainObject(value) {
    return value.toString() === '[object Object]';
}

isPlainObject(actor);        // TypeError : value.toString is not a function

// this is because inside actor object toString property is a string instead of inherited method from prototype

The Map does not have any restrictions on the key names, we can use key names like toString, constructor etc. Here although actorMap object has a property named toString but the method toString( ) inherited from prototype of actorMap object works perfectly.

const actorMap= new Map();

actorMap.set('name', 'Harrison Ford');

actorMap.set('toString', 'Actor: Harrison Ford');

function isMap(value) {
  return value.toString() === '[object Map]';
}

console.log(isMap(actorMap));     // true

If we have a situation where user input creates keys then we must take those keys inside a Map instead of a plain object. This is because user may choose a custom field name like, toString, constructor etc. then such key names in a plain object can potentially break the code that later uses this object. So the right solution is to bind the user interface state to a map, there is no way to break the Map :

const userCustomFieldsMap= new Map([['color', 'blue'], ['size', 'medium'], ['toString', 'A blue box']]);

3. Map is iterable :

To iterate a plain object's properties we need Object.entries( ) or Object.keys( ). The Object.entries(plainObject) returns an array of key value pairs extracted from the object, we can then destructure those keys and values and can get normal keys and values output.

const colorHex= {
  'white': '#FFFFFF',
  'black': '#000000'
}

for(const [color, hex] of Object.entries(colorHex)) {
  console.log(color, hex);
}
//
'white' '#FFFFFF'   
'black' '#000000'

As Maps are iterable that's why we do not need entries( ) methods to iterate over a Map and destructuring of key, value array can be done directly on the Map as inside a Map each element lives as an array of key value pairs separated by commas.

const colorHexMap= new Map();
colorHexMap.set('white', '#FFFFFF');
colorHexMap.set('black', '#000000');


for(const [color, hex] of colorHexMap) {
  console.log(color, hex);
}
//'white' '#FFFFFF'   'black' '#000000'

Also map.keys( ) returns an iterator over keys and map.values( ) returns an iterator over values.

4. We can easily know the size of a Map

We cannot directly determine the number of properties in a plain object. We need a helper fn like, Object.keys( ) which returns an array with keys of the object then using length property we can get the number of keys or the size of the plain object.

const exams= {'John Rambo': '80%', 'James Bond': '60%'};

const sizeOfObj= Object.keys(exams).length;

console.log(sizeOfObj);       // 2

But in the case of Maps we can have direct access to the size of the Map using map.size property.

const examsMap= new Map([['John Rambo', '80%'], ['James Bond', '60%']]);

console.log(examsMap.size);

This is a short way for me to remember it: KOI

1. Keys. Object key is strings or symbols. Map keys can also be numbers (1 and "1" are different), objects, NaN, etc. It uses === to distinguish between keys, with one exception NaN !== NaN but you can use NaN as a key.
2. Order. The insertion order is remembered. So [...map] or [...map.keys()] has a particular order.
3. Interface. Object: obj[key] or obj.a (in some language, [] and []= are really part of the interface). Map has get(), set(), has(), delete() etc. Note that you can use map[123] but that is using it as a plain JS object.

I don't think the following points have been mentioned in the answers so far, and I thought they'd be worth mentioning.

Maps can be bigger

In chrome I can get 16.7 million key/value pairs with Map vs. 11.1 million with a regular object. Almost exactly 50% more pairs with a Map. They both take up about 2GB of memory before they crash, and so I think may be to do with memory limiting by chrome (Edit: Yep, try filling 2 Maps and you only get to 8.3 million pairs each before it crashes). You can test it yourself with this code (run them separately and not at the same time, obviously):

var m = new Map();
var i = 0;
while(1) {
    m.set(((10**30)*Math.random()).toString(36), ((10**30)*Math.random()).toString(36));
    i++;
    if(i%1000 === 0) { console.log(i/1000,"thousand") }
}
// versus:
var m = {};
var i = 0;
while(1) {
    m[((10**30)*Math.random()).toString(36)] = ((10**30)*Math.random()).toString(36);
    i++;
    if(i%1000 === 0) { console.log(i/1000,"thousand") }
}

Objects have some properties/keys already

This one has tripped me up before. Regular objects have toString, constructor, valueOf, hasOwnProperty, isPrototypeOf and a bunch of other pre-existing properties. This may not be a big problem for most use cases, but it has caused problems for me before.

Maps can be slower:

Due to the .get function call overhead and lack of internal optimisation, Map can be considerably slower than a plain old JavaScript object for some tasks.

Summary:

• Object: A data structure in which data is stored as key value pairs. In an object the key has to be a number, string, or symbol. The value can be anything so also other objects, functions etc. A object is an non ordered data structure, i.e. the sequence of insertion of key value pairs is not remembered
• ES6 Map: A data structure in which data is stored as key value pairs. In which a unique key maps to a value. Both the key and the value can be in any data type. A map is a iterable data structure, this means that the sequence of insertion is remembered and that we can access the elements in e.g. a for..of loop

Key differences:

• A Map is ordered and iterable, whereas a objects is not ordered and not iterable

• We can put any type of data as a Map key, whereas objects can only have a number, string, or symbol as a key.

• A Map inherits from Map.prototype. This offers all sorts of utility functions and properties which makes working with Map objects a lot easier.

Example:

object:

let obj = {};_x000D_
_x000D_
// adding properties to a object_x000D_
obj.prop1 = 1;_x000D_
obj[2]    =  2;_x000D_
_x000D_
// getting nr of properties of the object_x000D_
console.log(Object.keys(obj).length)_x000D_
_x000D_
// deleting a property_x000D_
delete obj[2]_x000D_
_x000D_
console.log(obj)

Map:

const myMap = new Map();_x000D_
_x000D_
const keyString = 'a string',_x000D_
    keyObj = {},_x000D_
    keyFunc = function() {};_x000D_
_x000D_
// setting the values_x000D_
myMap.set(keyString, "value associated with 'a string'");_x000D_
myMap.set(keyObj, 'value associated with keyObj');_x000D_
myMap.set(keyFunc, 'value associated with keyFunc');_x000D_
_x000D_
console.log(myMap.size); // 3_x000D_
_x000D_
// getting the values_x000D_
console.log(myMap.get(keyString));    // "value associated with 'a string'"_x000D_
console.log(myMap.get(keyObj));       // "value associated with keyObj"_x000D_
console.log(myMap.get(keyFunc));      // "value associated with keyFunc"_x000D_
_x000D_
console.log(myMap.get('a string'));   // "value associated with 'a string'"_x000D_
                         // because keyString === 'a string'_x000D_
console.log(myMap.get({}));           // undefined, because keyObj !== {}_x000D_
console.log(myMap.get(function() {})) // undefined, because keyFunc !== function () {}

source: MDN

Additionally to being iterable in a well-defined order, and the ability to use arbitrary values as keys (except -0), maps can be useful because of the following reasons:

• The spec enforces map operations to be sublinear on average.

  Any non-stupid implementation of object will use a hash table or similar, so property lookups will probably be constant on average. Then objects could be even faster than maps. But that is not required by the spec.

• Objects can have nasty unexpected behaviors.

  For example, let's say you didn't set any foo property to a newly created object obj, so you expect obj.foo to return undefined. But foo could be built-in property inherited from Object.prototype. Or you attempt to create obj.foo by using an assignment, but some setter in Object.prototype runs instead of storing your value.

  Maps prevent these kind of things. Well, unless some script messes up with Map.prototype. And Object.create(null) would work too, but then you lose the simple object initializer syntax.

In addition to the other answers, I've found that Maps are more unwieldy and verbose to operate with than objects.

obj[key] += x
// vs.
map.set(map.get(key) + x)

This is important, because shorter code is faster to read, more directly expressive, and better kept in the programmer's head.

Another aspect: because set() returns the map, not the value, it's impossible to chain assignments.

foo = obj[key] = x;  // Does what you expect
foo = map.set(key, x)  // foo !== x; foo === map

Debugging maps is also more painful. Below, you can't actually see what keys are in the map. You'd have to write code to do that.

Objects can be evaluated by any IDE:

According to Mozilla

Object vs Map in JavaScript in short way with examples.

Object- follows the same concept as that of map i.e. using key-value pair for storing data. But there are slight differences which makes map a better performer in certain situations.

Map- is a data structure which helps in storing the data in the form of pairs. The pair consists of a unique key and a value mapped to the key. It helps prevent duplicity.

Key differences

• The Map is an instance of an object but the vice-versa is not true.

var map = new Map();_x000D_
var obj = new Object(); _x000D_
console.log(obj instanceof Map);   // false_x000D_
console.log(map instanceof Object);  // true

• In Object, the data-type of the key-field is restricted to integer, strings, and symbols. Whereas in Map, the key-field can be of any data-type (integer, an array, an object)

var map = new Map();//Empty _x000D_
map.set(1,'1');_x000D_
map.set('one', 1);_x000D_
map.set('{}', {name:'Hello world'});_x000D_
map.set(12.3, 12.3)_x000D_
map.set([12],[12345])_x000D_
_x000D_
for(let [key,value] of map.entries())_x000D_
  console.log(key+'---'+value)

• In the Map, the original order of elements is preserved. This is not true in case of objects.

let obj ={_x000D_
  1:'1',_x000D_
  'one':1,_x000D_
  '{}': {name:'Hello world'},_x000D_
  12.3:12.3,_x000D_
  [12]:[100]_x000D_
}_x000D_
console.log(obj)

An object behaves like a dictionary because Javascript is dynamically typed, allowing you to add or remove properties at any time.

But Map() is much better because it:

• Provides get, set, has, and delete methods.
• Accepts any type for the keys instead of just strings.
• Provides an iterator for easy for-of usage and maintains order of results.
• Doesn't have edge cases with prototypes and other properties showing up during iteration or copying.
• Supports millions of items.
• Is very fast.

If you need a dictionary then you should just use a Map().

However, if you're only using string-based keys and need maximum read performance, then objects might be a better choice. This is because Javascript engines compile objects down to C++ classes in the background. The access path for properties on these classes is very optimized and much faster than a function call for Map().get().

These classes are also cached, so creating a new object with the same exact properties means the engine will reuse an existing background class. Adding or removing a property causes the shape of the class to change and the backing class to be re-compiled, which is why using an object as a dictionary with lots of additions and deletions is very slow, but reads of existing keys without changing the object are very fast.

So if you have a write-once read-heavy workload with string keys then you can use an object as a high-performance dictionary, but for everything else use a Map().