[Edit]

Node.js - read process.stdin using fs.readSync() function (sync reading)

1 contributors

2 contributions

0 discussions

8 points

Created by:

Saim-Mccullough

718

In this short article, we would like to show a simple way how to read data from process.stdin using fs.readSync() function in Node.js.

Quick solution:

xxxxxxxxxx
 
const fs = require('fs');  // or: require('node:fs')
​
const buffer = Buffer.allocUnsafe(1024);
const count = fs.readSync(process.stdin.fd, buffer, 0, buffer.length);

Where:

buffer contains read bytes from process.stdin,
readSync() reads bytes and puts them to buffer starting from 0 index in buffer,
count contains number of read bytes.

Practical example

Edit

In this section you can find simple reusable function that lets to read exact number of bytes from process.stdin.

xxxxxxxxxx
 
const fs = require('fs');  // or: require('node:fs')
​
const readBytes = (fd, count) => {
    const buffer = Buffer.allocUnsafe(count);
    for (let i = 0; i < count;) {
        let result = 0;
        try {
            result = fs.readSync(fd, buffer, i, count - i);
        } catch (error) {
            if (error.code === 'EAGAIN') {  // when there is nothing to read at the current time (Unix)
                //TODO: it is good to slow down loop here or do other tasks in meantime, e.g. async sleep
                continue;
            }
            throw error;
        }
        if (result === 0) {
            throw new Error('Input stream reading error.');  // consider to use your own solution on this case
        }
        i += result;
    }
    return buffer;
};
​
​
// Usage example:
​
const buffer = readBytes(process.stdin.fd, 1024);  // waits for bytes in stdin and reads them into buffer

Hint: consider to use async API or standard input events.

Legacy version

Edit

In this section, you can fund unified version that works the same way under Window and Unix/Linux.

xxxxxxxxxx
 
const fs = require('fs');
​
// Reads bytes into buffer returning -1 when transmission ended.
//
const readSync = (fd, buffer, offset, length) => {
    try {
        const result = fs.readSync(fd, buffer, offset, length);
        return result === 0 ? -1 : result;
    } catch (error) {
        if (error.code === 'EAGAIN') {  // when there is nothing to read at the current time 
            return 0;
        }
        throw error;
    }
};