// create a set with ["a","b","c"] content
var set = new Set("abc");
// same as 
var set = new Set(["a","b","c"]);
// new Set(iterable)
set.has("a")
// -> true
set.size
// -> 3
set.add("d")
set.delete("a")
set.forEach(function(){}) // iterate over entries, in order of insertion
set.entries() // get iterator of set
set.keys(); set.values(); // iterators for interoparability with Map
set.values === set.keys // 
set.clear() // remove all entries

var map = new Map();
var map = new Map([[2,"b"], [1, "a"]]) // -> Map { 2: "b", 1: "a" }
map.size // -> 2
map.get(2) // -> "b"
map.set(key, value); // set or overwrite key
map.delete(key);
map.clear(); // delete all keys
map.forEach(function(value,key,map){}) // iterate over entries in order of insertion
map.keys(); // iterator over all keys
map.values(); // iterator over all values
map.entries(); // iterator over all entries (pairs) 

// returns iterator function for the body of this 
function* generate(array){
  for(var value of array){
    yield value;  
  }
}

function generate(){
  var i = 0;
  return function(){
    i++;
    return i;
  }
}

var array = ["a","b"];
var iterator = generate(array);
// calling next always executes to the next yield keyword 
// and returns the value after yield (like return)
iterator.next(); // -> {done: false, value: "a"}
iterator.next(); // -> {done: false, value: "b"}
// when the function reaches the end, the done property is set to false
iterator.next(); // -> {done: true, value: undefined}

// This function should return a promise
async function getValue(key){
  var awaitedValue = null;
  try {
    awaitedValue = await fetchMyValue(key);
  } catch(e) { // error handling }
  return awaitedValue;
}