How to Open File in Node.js?

Node.js / November 17, 2022 / TopCode / 0

Introduction

Opening a file is a common programming technique. Every web application does a file open operation. Whenever a file read or file write operation executes eventually it does a file open call as well. Without file open operation, reading and writing to a file are not possible.

To open file in Node.js, We can use built-in fs module of Node.js. fs module provides different APIs to open a file in Node. In this post, we will learn about all the ways to open a file in Node.js

Asynchronous operation using fs.open()

fs.open() is a simplest way to open a file in Node.js. It is a asynchronous API, which does not blocks the event loop of Node.js. Syntax of fs.open() is as follows:

fs.open(path[, flags[, mode]], callback)

This method accept four parameters, described as follows:

• path: It holds the name of the file to read or the entire path if stored at other location.
• flags: The operation in which file has to be opened. Default flag is “r”
• mode: Sets the mode of file i.e. r-read, w-write, r+ -readwrite. It sets to default as readwrite. (0o666).
• callback: It is a callback function that is called after opening a file. It takes two parameters:
  • err: If any error occurs.
  • fd: Returns the file descriptor of the opened file. FD is an ineger value.

Following code snippet is basic example of file open in Node.js.

const fs = require('fs');
  
// Open file foo.txt in read mode
fs.open('foo.txt', 'r', function (err, f) {
  console.log('File opened successfully !');
});

Synchronous operation using fs.openSync()

fs.openSync() is alternative way to open a file but it is a synchronous API which blocks the event loop until the file open operation is completed. Syntax of fs.openSync() is as follows:

fs.openSync(path[, flags, mode])

This method accept three parameters, described as follows:

• path: It holds the name of the file to read or the entire path if stored at other location.
• flags: The operation in which file has to be opened. Default flag is “r”
• mode: Sets the mode of file i.e. r-read, w-write, r+ -readwrite. It sets to default as readwrite. (0o666).

Following code snippet uses fs.openSync()

const fs = require('fs');
  
const fd = fs.openSync("foo.txt", 'r');
  
// Prints output
console.log(fd);

fs.openSync() returns an integer value which denotes the file descriptor of opened file.

Following is the list of supported flag in fs module.

• 'a': Open file for appending. The file is created if it does not exist.
• 'ax': Like 'a' but fails if the path exists.
• 'a+': Open file for reading and appending. The file is created if it does not exist.
• 'ax+': Like 'a+' but fails if the path exists.
• 'as': Open file for appending in synchronous mode. The file is created if it does not exist.
• 'as+': Open file for reading and appending in synchronous mode. The file is created if it does not exist.
• 'r': Open file for reading. An exception occurs if the file does not exist.
• 'r+': Open file for reading and writing. An exception occurs if the file does not exist.
• 'rs+': Open file for reading and writing in synchronous mode. Instructs the operating system to bypass the local file system cache.This is primarily useful for opening files on NFS mounts as it allows skipping the potentially stale local cache. It has a very real impact on I/O performance so using this flag is not recommended unless it is needed.This doesn’t turn fs.open() or fsPromises.open() into a synchronous blocking call. If synchronous operation is desired, something like fs.openSync() should be used.
• 'w': Open file for writing. The file is created (if it does not exist) or truncated (if it exists).
• 'wx': Like 'w' but fails if the path exists.
• 'w+': Open file for reading and writing. The file is created (if it does not exist) or truncated (if it exists).
• 'wx+': Like 'w+' but fails if the path exists.

Conclusion

It is recommended to use an asynchronous function instead of synchronous. As synchronous function blocks the event loop and should not be used in the production environment.Thanks for reading.

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