Map

new Map([iterable])

var myMap = new Map(

var keyString = 'a string',
    keyObj = {},
    keyFunc = function() {};

// setting the values
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

// getting the values
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'"
                         // because keyString === 'a string'
myMap.get{}           // undefined, because keyObj !== {}
myMap.get(function() {}) // undefined, because keyFunc !== function () {}

var myMap = new Map(
myMap.set(NaN, 'not a number'

myMap.get(NaN // "not a number"

var otherNaN = Number('foo'
myMap.get(otherNaN // "not a number"

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
}
// Will show 2 logs; first with "0 = zero" and second with "1 = one"

var kvArray = [['key1', 'value1'], ['key2', 'value2']];

// Use the regular Map constructor to transform a 2D key-value Array into a map
var myMap = new Map(kvArray

myMap.get('key1' // returns "value1"

// Use the Array.from function to transform a map into a 2D key-value Array
console.log(Array.from(myMap) // Will show you exactly the same Array as kvArray

// Or use the keys or values iterators and convert them to an array
console.log(Array.from(myMap.keys()) // Will show ["key1", "key2"]