ECMAScript 6 规范虽然仍处于草稿形式,但有望为 JavaScript 程序员提供许多令人兴奋的新工具。Set
和 Map
等新类提供了处理特定类型集合的原生解决方案,for...of
语句则提供了一种优雅的替代方案,可用于迭代数据的传统方法。
Set
提供了一种跟踪一组项的方法,其中每个项最多只能出现一次。Map
提供了比之前使用 Object
将键与值相关联时更多的功能。使用 Map
时,键不必是字符串,并且您不必担心意外选择的键名称与 Object
的方法名称冲突。对原生 Map
和 Set
执行的查找操作是在常量时间内完成的,与模拟实现相比,效率更高。
以下示例演示了如何构建 Set
,以及如何使用 for...of
迭代其元素:
<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>
以下是相应的示例,展示了如何使用和迭代 Map
:
<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>
某些浏览器(例如 Chrome、Internet Explorer 和 Firefox)已添加对 Set
和 Map
的支持。借助原生支持和 es6-collections 或 es6-shim 等 polyfill 库,JavaScript 开发者现在就可以开始使用这些新集合类型进行构建。目前没有适用于 for...of
语句的 polyfill(不过可以通过 Traceur 转译支持),但 Chrome 和 Firefox 目前提供原生支持。
更新(2014 年 9 月):关联到其他 polyfill 选项 es6-shim