Buffer is ArrayBuffer?

Question

I'm trying to read the first 100 bytes of a file named example.txt using fs.read in this Node.JS code:

"use strict";

const fs = require("fs");

fs.open("example.txt", function(err, fd) {
    if(err) {
        console.error(err);
    } else {
        fs.read(fd, Buffer.alloc(100), function onFileRead(err, bytesRead, buffer) {
            if(err) {
                console.error(err);
            } else if(bytesRead >= buffer.byteLength) {
                console.log("BEHOLD:\n" + buffer.toString());
            } else {
                console.log(`Read progress: ${ bytesRead } bytes (${ bytesRead / buffer.length * 100 }%)`);
            }
        });
    }
});

But running the code generates this error:

TypeError [ERR_INVALID_ARG_TYPE]: The "buffer" argument must be an instance of Buffer, TypedArray, or DataView. Received an instance of ArrayBuffer
←[90m    at Object.read (fs.js:492:3)←[39m
    at C:\Users\...\bufferTest.js:9:12
←[90m    at FSReqCallback.oncomplete (fs.js:155:23)←[39m {
  code: ←[32m'ERR_INVALID_ARG_TYPE'←[39m
}

This error makes no sense to me. How can a buffer returned by Buffer.alloc not be an instance of Buffer? Replacing Buffer.alloc(100) with Buffer.alloc(100).buffer or new Uint8Array(100) doesn't fix the issue either. I'm currently using Node.JS version 14.17.6 (LTS).

Danvin · Accepted Answer · 2021-09-15 20:40:23Z

1

The Syntax is : fs.read(fd, [options,] callback) options : is an object, it has a property buffer which has a default value of Buffer.alloc(16384)

fs.open("example.txt", function(err, fd) {

if (err) {

    console.error(err);

} else {

    let readBuffer = Buffer.alloc(100);

   // See here  buffer is the name of a property in the options object which is passed to the read method in fs

    fs.read(fd, {
        buffer: readBuffer
    }, function onFileRead(err, bytesRead, buffer) {

        if (err) {

            console.error(err);

        } else if (bytesRead >= buffer.byteLength) {

            console.log("BEHOLD:\n" + buffer.toString());

        } else {

            console.log(`Read progress: ${ bytesRead } bytes (${ bytesRead / buffer.length * 100 }%)`);

        }

    });

}

});

answered Sep 15, 2021 at 20:40

Danvin

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Danvin Over a year ago

Read the documentation on nodejs.org, It works if all the parameters for the read function are explicitly mentioned like this : fs.read(fd,buffer,0,buffer.length,null, function onFileRead(err, bytesRead, buffer) { if(err) { console.error(err); } else if(bytesRead >= buffer.byteLength) { console.log("BEHOLD:\n" + buffer.toString()); } else { console.log(Read progress: ${ bytesRead } bytes (${ bytesRead / buffer.length * 100 }%)); } });

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