fs-readstream-seek

A fs.ReadStream that supports seeking to arbtrary locations within a file.

Note that this stream is only appropriate for files where positioned reads are supported. For abstract filesystem objects where you wish to do ordered asynhronous reads without specifying position (for example, FIFO devices), use fs.ReadStream instead.

USAGE

const ReadStream = require('fs-readstream-seek')
const s = new ReadStream('some-filename.db')
s.seek(123)
s.once('data', chunk => {
  console.log('the data at position 123 is %s', chunk)
})

API

Everything on fs.ReadStream is supported, plus:

• stream.seek(n) Seek to a position in the file. If the position is within the portion of the file that has already been read into memory, no new read is triggered, and the in-memory buffer is updated. If the position is beyond the end of the buffer, or before the beginning of the buffer, then the buffer is discarded a new fs.read() is made at the apporpriate location.

• stream.readPos Read-only indication of where in the file the next read() will occur at. This is always updated when stream.seek(n) is called.

  Note that this is not the position where the current buffer in a 'data' event was found, but rather the position where the next data chunk will be read from. You can, however, get that value by subtracting the chunk length from the stream.readPos value.

  stream.on('data', chunk => {
    console.error('position=%d data=%j',
      stream.readPos - chunk.length,
      chunk.toString())
  })

• stream.filePos Read-only indication of where the read buffer is currently filled up to, and thus where the next fs.read() will occur within the file. This may be updated by stream.seek(n), if necessary, and will naturally increase as more data is pulled into the buffer.

Caveat re Stream Conventions

By convention, when a Readable stream emits an 'end' event, it is an indication that no more data will be made available. Thus 'end' is always a single-time event per-stream. Likewise, close and open events on fs streams are generally unique in the lifetime of a stream.

However, when you seek to a new location within a file, it resets the EOF handling. If the end of the file was read into the buffer, and thus automatically closed, then it will be re-opened if necessary when your program calls stream.seek(n).

So you can do this to read a file and print to stdout repeatedly:

const ReadStream = require('fs-readstream-seek')
const s = new ReadStream('some-filename.txt')
s.on('end', _ => {
  s.seek(0)
})
s.on('data', c => {
  process.stdout.write(c)
})

In this case, end will be emitted every time the stream gets to the end of the data. When s.seek(0) is called, the file is re-opened and starts reading from the beginning again.

Because it's a very common convention, 'end' and 'close' events cause a readable.pipe(writable) chain to be disassembled. If this is a thing that your program will be triggering by seek()-ing backwards in the file after it has emitted 'end', then you are strongly advised not to pipe() that data anywhere, and instead consume it directly using 'data' events or read() method calls.