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FileSystemWritableFileStream: seek() method

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The seek() method of the FileSystemWritableFileStream interface updates the current file cursor offset to the position (in bytes) specified when calling the method.

Syntax

js

seek(position)

Parameters

position

A number specifying the byte position from the beginning of the file.

Return value

A Promise that returns undefined.

Exceptions

NotAllowedError DOMException

Returned if PermissionStatus.state is not granted.

TypeError

Returned if position is not a number or not defined.

Examples

The following asynchronous function opens the 'Save File' picker, which returns a FileSystemFileHandle once a file is selected. From this, a writable stream is created using the FileSystemFileHandle.createWritable() method.

Next, we write to the stream:

  1. A text string is written to the stream.
  2. The seek() method is used to put the cursor at the start of the stream.
  3. A second text string is written to the start of the stream, overwriting the first write.

The stream is then closed.

js

async function saveFile() {
  try {
    // create a new handle
    const newHandle = await window.showSaveFilePicker();

    // create a FileSystemWritableFileStream to write to
    const writableStream = await newHandle.createWritable();

    // write our file
    await writableStream.write("My first file content");
    await writableStream.seek(0);
    await writableStream.write("My second file content");

    // close the file and write the contents to disk.
    await writableStream.close();
  } catch (err) {
    console.error(err.name, err.message);
  }
}

If you run the above function and then open the resulting file created on disk, you should see the text "My second file content".

Specifications

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
seek 86 86 111 No 72 No No No 111 No No No

See also

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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/API/FileSystemWritableFileStream/seek