FileSystemDirectoryEntry: createReader() method
  The FileSystemDirectoryEntry interface's method
  createReader() returns a
  FileSystemDirectoryReader object which can be used to read the entries in
  the directory.
Syntax
js
createReader()
Parameters
None.
Return value
  A FileSystemDirectoryReader object which can be used to read the
  directory's entries.
Examples
  This example creates an async function called readDirectory(), which fetches all of
  the entries in the specified FileSystemDirectoryEntry and returns them in
  an array.
js
async function readDirectory(directory) {
  const dirReader = directory.createReader();
  const entries = [];
  while (true) {
    const results = await new Promise((resolve, reject) => {
      dirReader.readEntries(resolve, reject);
    });
    if (!results.length) {
      break;
    }
    for (const entry of results) {
      entries.push(entry);
    }
  }
  return entries;
}
This works by calling readEntries() repetitively to get all the entries in the directory, concatenating each batch to the array. When it returns an empty array, all entries have been read, and the loop ends.
Specifications
| Specification | 
|---|
| File and Directory Entries API  # dom-filesystemdirectoryentry-createreader  | 
Browser compatibility
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