length 是 Storage 接口的只读属性，返回一个整数，表示存储在 Storage 对象里的数据项（data items）数量。
语法
var aLength = storage.length;
返回值
一个整数。
示例
下面的函数添加三个数据项到当前域名的本地存储里面，然后返回本地存储里面数据项的数量：
function populateStorage() {
localStorage.setItem('bgcolor', 'yellow');
localStorage.setItem('font', 'Helvetica');
localStorage.setItem('image', 'cats.png');
localStorage.length; // 返回 3
}
备注： 关于实际的例子，可以查看 Web Storage Demo.
规范
| Specification | Status | Comment |
|---|---|---|
| Unknown length |
Unknown |
浏览器兼容性
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This compatibility table still uses the old format,
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| Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari (WebKit) |
|---|---|---|---|---|---|
| localStorage | 4 | 3.5 | 8 | 10.50 | 4 |
| sessionStorage | 5 | 2 | 8 | 10.50 | 4 |
| Feature | Android | Firefox Mobile (Gecko) | IE Phone | Opera Mobile | Safari Mobile |
|---|---|---|---|---|---|
| Basic support | 2.1 | ? | 8 | 11 | iOS 3.2 |
各浏览器支持的 localStorage 和 sessionStorage 容量不同。测试页面：detailed rundown of all the storage capacities for various browsers.
Note: since iOS 5.1, Safari Mobile stores localStorage data in the cache folder, which is subject to occasional clean up, at the behest of the OS, typically if space is short.