ECMAScript 6 規格仍處於草稿形式,但我們承諾會提供許多令人期待的新工具,讓 JavaScript 程式設計師能充分運用。Set
和 Map
等新類別提供原生解決方案,可處理特定類型的集合,而 for...of
陳述式則提供傳統資料疊代方法的優雅替代方案。
Set
可用於追蹤項目集合,其中每個項目最多只能出現一次。Map
透過使用 Object
將鍵與值建立關聯,提供比先前更豐富的功能。使用 Map
時,鍵不必是字串,也不必擔心不小心選擇與 Object
方法名稱相衝突的鍵名稱。原生 Map
和 Set
的查詢作業會在固定時間內完成,這比模擬實作更有效率。
以下範例示範如何建構 Set
,並使用 for...of
對其元素進行疊代:
<pre id="log"></pre>
<script>
function log() {
document.querySelector('#log').textContent += Array.prototype.join.call(arguments, '') + '\n';
}
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);
}
log(randomIntegers.size, ' distinct integers were generated.');
log('The number 10 was ', randomIntegers.has(10) ? '' : 'not ', 'one of them.');
log('Here\'s all of them:');
// Use for...of to iterate over the items in the Set.
// https://people.mozilla.org/~jorendorff/es6-draft.html#sec-iteration-statements
// The Set iterator yields a single value corresponding to each entry in the Set.
for (var item of randomIntegers) {
log(item);
}
</script>
以下是對應的範例,說明如何使用 Map
並對其進行疊代:
<script>
log('\nCreating and iterating over a Map:');
// Maps can be initialized by passing in an iterable value (https://people.mozilla.org/~jorendorff/es6-draft.html#sec-map-iterable)
// 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.
// https://people.mozilla.org/~jorendorff/es6-draft.html#sec-iteration-statements
// There are several types of Map iterators available.
// typesOfKeys.keys() can be used to iterate over just the keys:
log('Just the keys:');
for (var key of typesOfKeys.keys()) {
log(' key: ', key);
}
// typesOfKeys.values() can be used to iterate over just the values:
log('Just the values:');
for (var value of typesOfKeys.values()) {
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().
log('Keys and values:');
// Alternative, ES6-idiomatic syntax currently supported in Safari & Firefox:
// for (var [key, value] of typesOfKeys) { … }
for (var item of typesOfKeys) {
log(' ', item[0], ' -> ', item[1]);
}
</script>
Chrome、Internet Explorer 和 Firefox 等部分瀏覽器已支援 Set
和 Map
。原生支援功能搭配 es6-collections 或 es6-shim 等 polyfill 程式庫,這表示 JavaScript 開發人員現在就能開始使用這些新的集合類型進行建構。for...of
陳述式沒有可用的 polyfill (雖然可以透過 Traceur 轉譯支援),但目前 Chrome 和 Firefox 提供原生支援。
更新 (2014 年 9 月):已連結至其他 polyfill 選項 es6-shim