node.js: fs.readFile(path[, options], callback): Read file asynchronously

fs.readFile(path[, options], callback)

This method read the file asynchronously. ‘callback’ function is called with contents of the file.

Below table summarizes the arguments of readFile method.

Argument	Type	Description
path	string or Buffer or URL or Integer	Specifies the file path or file descriptor.
options	string or Object	Below table summarizes different options supported by readFile method. Option Type Description encoding string Default value is null. If no encoding is specified, raw buffer is returned. flag string Default value is 'r'. Following modes are supported by node.js. '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. Abc
callback	Function	Whenever file data is read, call back function is called with two arguments. Below table summarizes the arguments of callback function. Argument Type Description err Error On success scenarios, value of err is null, on failure cases, it represents the stack trace. data string or Buffer Data represents the contents of the file.

 Note

Since this method read all the content of the file to in-memory, use the method ‘fs.createReadStream()’, while working with big files.

Let’s say input.txt contains below information.

input.txt

Two roads diverged in a yellow wood,

Jb_modern_frost_2_eAnd sorry I could not travel both

And be one traveler, long I stood

And looked down one as far as I could

To where it bent in the undergrowth;

HelloWorld.js

var fs = require('fs');

var filePath = 'input.txt';

var contents = fs.readFile(filePath, 'utf-8', (err, data) => {

    if (err)

        return console.error(error);

    console.log(data);

});

console.log('Done.......')

Output

Done.......

Two roads diverged in a yellow wood,

Jb_modern_frost_2_eAnd sorry I could not travel both

And be one traveler, long I stood

And looked down one as far as I could

To where it bent in the undergrowth;

If you do not specify the encoding, readFile method returns the Buffer.

HelloWorld.js

var fs = require('fs');

var filePath = 'input.txt';

var contents = fs.readFile(filePath, (err, data) => {

    if (err)

        return console.error(error);

    console.log(data);

});

console.log('Done.......')

Output

Done.......

<Buffer 54 77 6f 20 72 6f 61 64 73 20 64 69 76 65 72 67 65 64 20 69 6e 20 61 20 79 65 6c 6c 6f 77 20 77 6f 6f 64 2c 0d 0a 4a 62 5f 6d 6f 64 65 72 6e 5f 66 72 ... >

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