On POSIX systems, for every process, the kernel maintains a table of currently open files and resources. Each open file is assigned a simple numeric identifier called a file descriptor. At the system-level, all file system operations use these file descriptors to identify and track each specific file. Windows systems use a different but conceptually similar mechanism for tracking resources. To simplify things for users, Node.js abstracts away the specific differences between operating systems and assigns all open files a numeric file descriptor.
The fs.open() method is used to allocate a new file descriptor. Once allocated, the file descriptor may be used to read data from, write data to, or request information about the file.
fs.open('app.js', 'r', (err, fd) => {
if (err) throw err;
fs.fstat(fd, (err, stat) => {
if (err) throw err;
// use stat
// always close the file descriptor!
fs.close(fd, (err) => {
if (err) trow err;
});
});
});###Synatx Following is the syntax of one of the methods to read from a file -
fs.read(fd, buffer, offset, length, position, callback)Parameters
Here is the description of the parameters used -
- fd - This is the file descriptor returned by fs.open().
- buffer - This is the buffer that the data will be written to.
- offset - This is the offset in the buffer to start writing at.
- length - This is an integer specifying the number of bytes to read.
- position - This is an integer specifying where to begin reading from in the file. If position is null, data will be read from the current file position.
- callback - This is the callback function which gets the three arguments, (err, bytesRead, buffer).