FileSystemDirectoryReader.readEntries() - Web APIs 编辑

Experimental

This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

Non-standard

This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.

The FileSystemDirectoryReader interface's readEntries() method retrieves the directory entries within the directory being read and delivers them in an array to a provided callback function. The objects in the array are all based upon FileSystemEntry. Generally, they are either FileSystemFileEntry objects, which represent standard files, or FileSystemDirectoryEntry objects, which represent directories.

Syntax

readEntries(successCallback[, errorCallback]);

Parameters

successCallback

A function which is called when the directory's contents have been retrieved. The function receives a single input parameter: an array of file system entry objects, each based on FileSystemEntry. Generally, they are either FileSystemFileEntry objects, which represent standard files, or FileSystemDirectoryEntry objects, which represent directories. If there are no files left, or you've already called readEntries() on this FileSystemDirectoryReader, the array is empty.

errorCallback Optional

A callback function which is called if an error occurs while reading from the directory. It receives one input parameter: a FileError object describing the error which occurred.

Return value

undefined.

Example

In this example, a drop zone is created, which responds to the drop event by scanning through the dropped files and directories, outputting a hierarchical directory listing.

HTML content

The HTML establishes the drop zone itself, which is a <div> element with the ID "dropzone", and an unordered list element with the ID "listing".

<p>Drag files and/or directories to the box below!</p>

<div id="dropzone">
  <div id="boxtitle">
    Drop Files Here
  </div>
</div>

<h2>Directory tree:</h2>

<ul id="listing">
</ul>

CSS content

The styles used by the example are shown here.

#dropzone {
  text-align: center;
  width: 300px;
  height: 100px;
  margin: 10px;
  padding: 10px;
  border: 4px dashed red;
  border-radius: 10px;
}

#boxtitle {
  display: table-cell;
  vertical-align: middle;
  text-align: center;
  color: black;
  font: bold 2em "Arial", sans-serif;
  width: 300px;
  height: 100px;
}

body {
  font: 14px "Arial", sans-serif;
}

JavaScript content

First, let's look at the recursive scanFiles() function. This function takes as input a FileSystemEntry representing an entry in the file system to be scanned and processed (the item parameter), and an element into which to insert the list of contents (the container parameter).

Note that to read all files in a directory, readEntries needs to be called repeatedly until it returns an empty array. In Chromium-based browsers, the following example will only return a max of 100 entries.

let dropzone = document.getElementById("dropzone");
let listing = document.getElementById("listing");

function scanFiles(item, container) {
  let elem = document.createElement("li");
  elem.textContent = item.name;
  container.appendChild(elem);

 if (item.isDirectory) {
    let directoryReader = item.createReader();
    let directoryContainer = document.createElement("ul");
    container.appendChild(directoryContainer);
    directoryReader.readEntries(function(entries) {
        entries.forEach(function(entry) {
          scanFiles(entry, directoryContainer);
      });
    });
  }
}

scanFiles() begins by creating a new <li> element to represent the item being scanned, inserts the name of the item into it as its text content, and then appends it to the container. The container is always a list element in this example, as you'll see shortly.

Once the current item is in the list, the item's isDirectory property is checked. If the item is a directory, we need to recurse into that directory. The first step is to create a FileSystemDirectoryReader to handle fetching the directory's contents. That's done by calling the item's createReader() method. Then a new <ul> is created and appended to the parent list; this will contain the directory's contents in the next level down in the list's hierarchy.

After that, directoryReader.readEntries() is called to read in all the entries in the directory. These are each, in turn, passed into a recursive call to scanFiles() to process them. Any of them which are files are inserted into the list; any which are directories are inserted into the list and a new level of the list's hierarchy is added below, and so forth.

Then come the event handlers. First, we prevent the dragover event from being handled by the default handler, so that our drop zone can receive the drop:

dropzone.addEventListener("dragover", function(event) {
    event.preventDefault();
}, false);

The event handler that kicks everything off, of course, is the handler for the drop event:

dropzone.addEventListener("drop", function(event) {
  let items = event.dataTransfer.items;

  event.preventDefault();
  listing.textContent = "";

  for (let i=0; i<items.length; i++) {
    let item = items[i].webkitGetAsEntry();

    if (item) {
        scanFiles(item, listing);
    }
  }
}, false);

This fetches the list of DataTransferItem objects representing the items dropped from event.dataTransfer.items. Then we call Event.preventDefault() to prevent the event from being handled further after we're done.

Now it's time to start building the list. First, the list is emptied by setting listing.textContent to be empty. That leaves us with an empty ul to begin inserting directory entries into.

Then we iterate over the items in the list of dropped items. For each one, we call its webkitGetAsEntry() method to obtain a FileSystemEntry representing the file. If that's successful, we call scanFiles() to process the item—either by adding it to the list if it's just a file or by adding it and walking down into it if it's a directory.

Result

You can see how this works by trying it out below. Find some files and directories and drag them in, and take a look at the resulting output.

Specifications

This API has no official W3C or WHATWG specification.

Browser compatibility

BCD tables only load in the browser

On Chrome 77, readEntries() will only return the first 100 FileSystemEntry instances. In order to obtain all of the instances, readEntries() must be called multiple times.

See also

• File and Directory Entries API
• Introduction to the File System API
• FileSystemDirectoryEntry
• FileSystem