Maps (ES2015)

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.

> var animalSounds = new Map();
> animalSounds.set("dog", "woof");
Map {"dog" => "woof"}
> animalSounds.set("cat", "meow");
Map {"dog" => "woof", "cat" => "meow"}
> animalSounds.set("frog", "ribbit");
Map {"dog" => "woof", "cat" => "meow", "frog" => "ribbit"}

> console.log(animalSounds.size);
3
> console.log(animalSounds.has("dog"));
true

> animalSounds.delete("dog");
true

> console.log(animalSounds.size);
2
> console.log(animalSounds.has("dog"));
false

> animalSounds.clear();

> console.log(animalSounds.size);
0

Objects are similar to Maps in that both let you...

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

However, there are important differences between Objects and Maps that make using a Map better:

An Object has a prototype, so there are default keys in the map. Maps doesn't have default values
The keys of an Object are Strings and Symbols, where they can be any value for a Map (other objects for example).
You can get the size of a Map easily while you have to manually keep track of size for an Object.

When should i use a map?

Signs that you want a Map for a collection:

Are keys usually unknown until run time, do you need to look them up dynamically?
Do all values have the same type, and can be used interchangeably?
Do you need keys that aren't strings?
Are key-value pairs often added or removed?
Do you have an arbitrary (easily changing) amount of key-value pairs? Is the collection iterated?

If in contrast you have a fixed amount of keys, operate on them individually, and distinguish between their usage, then you want an object.

> var myMap = new Map();
> var keyString = "a string",
    keyObj = {},
    keyFunc = function () {};

> myMap.set(keyString, "value associated with 'a string'");
> myMap.set(keyObj, "value associated with keyObj");
> myMap.set(keyFunc, "value associated with keyFunc");
> myMap.size;
3

> myMap.get(keyString);
"value associated with 'a string'"
> myMap.get(keyObj);
"value associated with keyObj"
> myMap.get(keyFunc); 
"value associated with keyFunc"

> myMap.get("a string");
"value associated with 'a string'"
> myMap.get({}); // undefined, because keyObj !== {}
undefined
> myMap.get(function() {}) // undefined, because keyFunc !== function () {}
undefined

Iterating over Maps

> var myMap = new Map();
> myMap.set(0, "zero");
> myMap.set(1, "one");

> for (var [key, value] of myMap) {
  console.log(key + " = " + value);
}
0 = zero
1 = one

> for (var key of myMap.keys()) {
  console.log(key);
}
0
1

> for (var value of myMap.values()) {
  console.log(value);
}
zero
one

> for (var [key, value] of myMap.entries()) {
  console.log(key + " = " + value);
}
0 = zero
1 = one

> myMap.forEach(function(value, key) {
  console.log(key + " = " + value);
}, myMap)
0 = zero
1 = one

More info:

juanmaguitar/maps-es2015.md

Maps (ES2015)

When should i use a map?

Iterating over Maps