Collections and Iterators (ES6) Sample

Available in Chrome 38+ | View on GitHub | Browse Samples


There are several new types of collections and ways of iterating over collections in the draft ECMAScript 6 specification.

This sample demonstrates creating and working with two types of collections, Set and Map. It also demonstrates using the for...of syntax to iterate over each type of collection.

Live Output

JavaScript Snippet

ChromeSamples.log('Creating, using, and iterating over a Set:');
var randomIntegers = new Set();
// Generate a random integer in the range [1..10] five times,
// and use a Set to keep track of the distinct integers that were generated.
for (var i = 0; i < 5; i++) {
  randomIntegers.add(Math.floor(Math.random() * 10) + 1);

ChromeSamples.log(randomIntegers.size, 'distinct integers were generated.');
ChromeSamples.log('The number 10 was ' +
  (randomIntegers.has(10) ? '' : 'not ') + 'one of them.');
ChromeSamples.log('Here\'s all of them:');

// Use for...of to iterate over the items in the Set.
// The Set iterator yields a single value corresponding to each entry in the Set.
for (var integer of randomIntegers) {

ChromeSamples.log('\nCreating and iterating over a Map:');
// Maps can be initialized by passing in an iterable value (
// Here, we use an Array of Arrays to initialize. The first value in each sub-Array is the new
// Map entry's key, and the second is the item's value.
var typesOfKeys = new Map([
  ['one', 'My key is a string.'],
  ['1', 'My key is also a string'],
  [1, 'My key is a number'],
  [document.querySelector('#log'), 'My key is an object']
// You can also call set() to add new keys/values to an existing Map.
typesOfKeys.set('!!!!', 'My key is excited!');

// Use for...of to iterate over the items in the Map.
// There are several types of Map iterators available.
// typesOfKeys.keys() can be used to iterate over just the keys:
ChromeSamples.log('Just the keys:');
for (var key of typesOfKeys.keys()) {
  ChromeSamples.log('  key: ', key);

// typesOfKeys.values() can be used to iterate over just the values:
ChromeSamples.log('Just the values:');
for (var value of typesOfKeys.values()) {
  ChromeSamples.log('  value: ', value);

// The default Map iterator yields an Array with two items; the first is the Map entry's key and the
// second is the Map entry's value. This default iterator is equivalent to typesOfKeys.entries().
ChromeSamples.log('Keys and values:');
// Alternative, ES6-idiomatic syntax currently supported in Safari & Firefox:
// for ([key, value] of typesOfKeys) { ... }
for (var item of typesOfKeys) {
  ChromeSamples.log('  ', item[0], ' -> ', item[1]);