Javascript 如何在不获取全部信息的情况下获取 Firebase 中元素/列表的大小？
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How to get size of an element/list in Firebase without get it all?
提问by Rodolfo Paranhos
I need to measure the size/length of an element or list in Firebase but I don't need the real content.
我需要在 Firebase 中测量元素或列表的大小/长度，但我不需要真正的内容。
Something like firebase.database().ref("users/").once("length" || "size").then(callback)
就像是 firebase.database().ref("users/").once("length" || "size").then(callback)
psJavascript SDK
psJavascript SDK
回答by Frank van Puffelen
Firebase doesn't have a count operator, so the only way is to download all children orkeep a separate <children>_countproperty in sync. The latter is not a trivial task (see my answer here for one approachand this example Cloud Function), so most often developers likely end up going with the downloads-too-much-data-but-is-trivial approach:
Firebase 没有计数运算符，因此唯一的方法是下载所有子项或保持单独的<children>_count属性同步。后者不是一项微不足道的任务（请参阅我在此处的答案以了解一种方法和此示例 Cloud Function），因此大多数情况下，开发人员最终可能会采用下载过多数据但很简单的方法：
ref.child("messages").on("value", function(snapshot) {
console.log("There are "+snapshot.numChildren()+" messages");
})
A more efficient way to count the children would be to fire a REST call with shallow=trueparameter, which will give you just the keys. See In Firebase, is there a way to get the number of children of a node without loading all the node data?
计算子项的更有效方法是使用shallow=true参数触发 REST 调用，这将只为您提供密钥。请参阅在 Firebase 中，有没有办法在不加载所有节点数据的情况下获取节点的子节点数？
回答by Aleks Witko
Also found a post which does it a different way...
还发现了一个帖子，它以不同的方式...
Object.keys(users).length;
Post: Length of a JavaScript object
Post：JavaScript 对象的长度
