node.js: readSync from stdin?

user_277767 0
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Is it possible to synchronously read from stdin in node.js? Because I'm writing a brainfuck to JavaScript compiler in JavaScript (just for fun). Brainfuck supports a read operation which needs to be implemented synchronously.

I tried this:

const fs = require('fs');
var c = fs.readSync(0,1,null,'utf-8');
console.log('character: '+c+' ('+c.charCodeAt(0)+')');

But this only produces this output:

fs:189
  var r = binding.read(fd, buffer, offset, length, position);
              ^
Error: EAGAIN, Resource temporarily unavailable
    at Object.readSync (fs:189:19)
    at Object.<anonymous> (/home/.../stdin.js:3:12)
    at Module._compile (module:426:23)
    at Module._loadScriptSync (module:436:8)
    at Module.loadSync (module:306:10)
    at Object.runMain (module:490:22)
    at node.js:254:10
node.js synchronous stdin
11 Answers

Here is the implementation with `async await`. In the below code, the input is taken from standard input and after receiving data the standard input is stopped waiting for data by using `process.stdin.pause();`.

process.stdin.setEncoding('utf8');

// This function reads only one line on console synchronously. After pressing `enter` key the console will stop listening for data.
function readlineSync() {
    return new Promise((resolve, reject) => {
        process.stdin.resume();
        process.stdin.on('data', function (data) {
            process.stdin.pause(); // stops after one line reads
            resolve(data);
        });
    });
}

// entry point
async function main() {
    let inputLine1 = await readlineSync();
    console.log('inputLine1 = ', inputLine1);
    let inputLine2 = await readlineSync();
    console.log('inputLine2 = ', inputLine2);
    console.log('bye');
}

main();
user_70512990

The following code reads sync from stdin. Input is read up until a newline / enter key. The function returns a string of the input with line feeds (\n) and carriage returns (\r) discarded. This should be compatible with Windows, Linux, and Mac OSX. Added conditional call to Buffer.alloc (new Buffer(size) is currently deprecated, but some older versions lack Buffer.alloc.

function prompt(){
    var fs = require("fs");

    var rtnval = "";

    var buffer = Buffer.alloc ? Buffer.alloc(1) : new Buffer(1);

    for(;;){
        fs.readSync(0, buffer, 0, 1);   //0 is fd for stdin
        if(buffer[0] === 10){   //LF \n   return on line feed
            break;
        }else if(buffer[0] !== 13){     //CR \r   skip carriage return
            rtnval += new String(buffer);
        }
    }

    return rtnval;
}
user_34794500
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