node.js: fs.createReadStream(path[, options]): Create readable stream

fs.createReadStream(path[, options])

This method creates a readable stream. In my previous posts, I explained how to read a file using readFile and readFileSync methods. One of the disadvantage of these methods is, these methods read the entire content of the file to in-memory. In case of big files, your application will endup in lot of memory consumption. To resolve the memory issues, you can create a readable stream to the file and read chuck by chucnk.

Below table summarizes the arguments of createReadStream method.

Argument	Type	Description
path	string or Buffer or URL	Specifies the path of the file.
options	string or Object.	Below table summarizes different options supported by createReadStream method. Option Type Description flags string Default value is r. encoding string Default value is null fd integer Default value is null. If fd is specified and start is omitted or undefined, fs.createReadStream() reads sequentially from the current file position. If fd is specified, ReadStream will ignore the path argument and will use the specified file descriptor. mode integer Default value is 0o666 autoclose boolean Default value is true. If it is set true, then the stream is closed. If it is set to false, then it is application responsibility to close the stream. start integer Start byte of the stream to read. By using ‘start’ and ‘end’ bytes, you can read specific portion of the file. Example fs.createReadStream('sample.txt', { start: 10, end: 29 }); Above statement read 30 (from 10th byte to 29th byte) bytes from the file. end integer End byte of the stream. Example fs.createReadStream('sample.txt', { start: 10, end: 29 }); Above statement read 30 (from 10th byte to 29th byte) bytes from the file. highWaterMark integer Specifies the chuck size. Default value is 64 * 1024 (64kb). A

Node.js support below flags.

a.   'a' - Open file for appending. The file is created if it does not exist.

b.   'ax' - Like 'a' but fails if the path exists.

c.   'a+' - Open file for reading and appending. The file is created if it does not exist.

d.   'ax+' - Like 'a+' but fails if the path exists.

e.   'as' - Open file for appending in synchronous mode. The file is created if it does not exist.

f.    'as+' - Open file for reading and appending in synchronous mode. The file is created if it does not exist.

g.   'r' - Open file for reading. An exception occurs if the file does not exist.

h.   'r+' - Open file for reading and writing. An exception occurs if the file does not exist.

i.     'rs+' - Open file for reading and writing in synchronous mode. Instructs the operating system to bypass the local file system cache.

j.    'w' - Open file for writing. The file is created (if it does not exist) or truncated (if it exists).

k.   'wx' - Like 'w' but fails if the path exists.

l.     'w+' - Open file for reading and writing. The file is created (if it does not exist) or truncated (if it exists).

m. 'wx+' - Like 'w+' but fails if the path exists.

Below table summarizes the modes supported by node.js

Constant (mode)	Octal value	Description
fs.constants.S_IRUSR	0o400	read by owner
fs.constants.S_IWUSR	0o200	write by owner
fs.constants.S_IXUSR	0o100	execute/search by owner
fs.constants.S_IRGRP	0o40	read by group
fs.constants.S_IWGRP	0o20	write by group
fs.constants.S_IXGRP	0o10	execute/search by group
fs.constants.S_IROTH	0o4	read by others
fs.constants.S_IWOTH	0o2	write by others
fs.constants.S_IXOTH	0o1	execute/search by others

Find the below working application.

HelloWorld.js

var fs = require('fs');

var readStream = fs.createReadStream('data.txt', 'UTF-8');

readStream.once('data', (chunk) => {
    console.log('Started reading the content\n');
});

readStream.on('data', (chunk) => {
    console.log('*********************');
    console.log(`Reading ${chunk.length} bytes`);
    //console.log(chunk); // Uncomment this line to print tghe data
    console.log('*********************\n');
});

readStream.on('end', () => {
    console.log('*********************');
    console.log('Finished reading');
    console.log('*********************\n');
});

readStream.on('close', () => {
    console.log('*********************');
    console.log('Closing the stream');
    console.log('*********************');
});

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