The following four data structures are new in ECMAScript 6: Map, WeakMap, Set and WeakSet.

The Map object is a simple key/value map. Any value (both objects and primitive values) may be used as either a key or a value.

Basic usage

var colorsMap = new Map();
console.log(colorsMap.get('redKey')); // red
console.log(colorsMap.has('redKey')); // true
console.log(colorsMap.has('redKey')); // false

Map size and clear:

var map = new Map([
    [ 1, 'one' ],
    [ 2, 'two' ],
    [ 3, 'three' ]
console.log(map.size); // 3
console.log(map.size); // 0

Iterating Maps with for..of

var map = new Map();
map.set(1, "One");
map.set(2, "two");
map.set(3, "three");
for(let [key,value] of map) {
    console.log(`Key: ${key} Value: ${value}`);

Objects Vs maps

Objects allow you to set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key.

Map objects, however, have a few more advantages that make them better maps.

  • The keys of an Object are Strings, where they can be of any value for a Map.
  • You can get the size of a Map easily while you have to manually keep track of size for an Object.
  • The iteration of maps is in insertion order of the elements.
  • An Object has a prototype, so there are default keys in the map. (this can be bypassed using map = Object.create(null)).

These three 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 whether it's a number value, boolean value or any other primitive value.
  • Use objects when there is logic that operates on individual elements.