The version 6 of EcmaScript has introduced a lot of things to JavaScript and one of the most important one is Map, which is a very popular data structure in other programming languages. Though the plain old JavaScript objects looks pretty similar to what map is; but the actual Map has been introduced in ES6 only.

Map

What is Map?

In simple words, map means relating one thing to another. That one thing is called key and the another is value. Both key and value can be of any type. The major difference of Map from JavaScript Object is; objects accepts only String or Symbol key, but Map accepts anything. Below is a simple example of how to use ES6 Map.

Map examples

//instantiating a map
var myMap = new Map();
//set things in Map 
myMap.set(somethingAsKey, somethingElseAsValue);
//get the value back from Map
map.get(somethingAsKey);

/* Some more examples */
var dummyObject = {a:1};
var dummyFunc = function () {
    // body...
};

var yourMap = new Map();

yourMap.set("person", { name: "Paul Shan" });
yourMap.set(13, "Unlucky number");
yourMap.set(null, "I am null");

yourMap.get("person"); //{ name: "Paul Shan" }
yourMap.get(13); //Unlucky number
yourMap.get(null); //I am null

yourMap.set(undefined, "I am undefined");
yourMap.get(undefined); //I am undefined

yourMap.set({a:1}, "I am for new object");
yourMap.get({a:1}); //undefined

yourMap.set(dummyObject, "I am for dummy object");
yourMap.get(dummyObject); //I am for dummy object

yourMap.set(dummyFunc, "I am for dummy function");
yourMap.get(dummyFunc); //I am for dummy function


var theirMap = new Map([
    ["president", "David Palmer"],
    ["country", "United States"]
]);

theirMap.get("president"); //David Palmer
theirMap.get("country"); //United States

Hope the examples above has cleared your confusion regarding the differences between Map and Object to a certain extent. But the differences are not limited to getters, setters or type of keys. More differences are given below.

ES6 Map vs Object

You can learn more about Map in MDN

WeakMap

What is a WeakMap

WeakMap was also introduced by ES6 in 2015. By the name we can guess that it’s some special kind of Map may be; but actually it’s not. WeakMap is not inherited from Map. However their functionalities are very much similar.

The differences which Map has with Objects; are also true (majorly) for WeakMap. The major difference between Map and WeakMap is, WeakMap holds the references of the key objects weakly. Thus if that object is deleted somewhere in the program, WeakMap will also release the value mapped to that object. This prevents memory leak. This is the reason WeakMap is used to declare private variables. However there are more differenced between Map & WeakMap.

Map vs WeakMap

Below are some examples of WeakMaps. To know more about WeakMaps, you can check MDN document.

WeakMap examples

//instantiating a weakWeakMap
var myWeakMap = new WeakMap();
//set things in WeakMap 
myWeakMap.set(someObjectAsKey, somethingElseAsValue);
//get the value back from WeakMap
weakWeakMap.get(someObjectAsKey);

/* Some more examples */
var dummyObject = {a:1};
var dummyFunc = function () {
    // body...
};

var yourWeakMap = new WeakMap();


yourWeakMap.set({a:1}, "I am for new object");
yourWeakMap.get({a:1}); //undefined

yourWeakMap.set(dummyObject, "I am for dummy object");
yourWeakMap.get(dummyObject); //I am for dummy object

//ERRORS
yourWeakMap.set(dummyFunc, "I am for dummy function"); //ERROR
yourWeakMap.set("person", { name: "Paul Shan" }); // ERROR

var theirMap = new Map([
    [dummyObject, "David Palmer"]
]);

theirMap.get(dummyObject); //David Palmer