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

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:

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

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

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

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

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;
    }
};

See also

1. Node.js - write buffer into process.stdout using fs.writeSync() function

References

1. process.stdin - Node.js API
2. fs.readSync() - Node.js API