Nodejs readable.read() return null

Question

I want read bytes from a file, here is the sample code:

var fs = require('fs');

var stream= fs.createReadStream('./lib');
console.log(stream.read(10));

return null.

I think it's because read() method directly get data from the inner buffer. If the buffer has not enough data, the null is returned. The read() method is a sync call, no callback being passed, so that design makes sense. However I don't know when the inner buffer can have enough data so that my read() call can return data since it's paused mode now?

Update1:

event readable is a good way to get started. But if I want to read a large file, and the read count > 65536, null will return.

var fs = require('fs');

var stream = fs.createReadStream('./lib');
stream.on('readable', function () {
    var buffer = stream.read(65537);
    console.log(buffer.length);

});

What I want exactly is stream.read() can always return data till the EOF.

Answer 1

Wait for the readable event to fire before trying to read:

var fs = require('fs');

var stream= fs.createReadStream('./lib');
stream.on('readable', function () {
  var buffer = stream.read(10);
  if (buffer) {
    console.log(buffer.toString());
  }
});

Answer 2

According to the official document:

Once the internal buffer is drained, a 'readable' event will fire again when more data is available.

I can use async/await so that readable.read() can always return data as long as not at the EOF.

let rs = fs.createReadStream('./resources/1.gif');

async function readable(): Promise<{}> {
    return new Promise(r => rs.on('readable', r));
}

async function readBytes(num: number = 0): Promise<Buffer> {
    let buf = rs.read(num);
    if (buf) {
        return new Promise<Buffer>(r => r(buf));
    }
    else {
        return new Promise<Buffer>(r => {
            this.readable().then(() => {
                this.readBytes(num).then(b => r(b));
            });
        });
    }
}

async function main() {
    console.log('begin');
    console.log((await readBytes(10)).length);
    console.log((await readBytes(65535)).length);
    console.log((await readBytes(100000)).length);
    console.log((await readBytes(10)).length);
    console.log('end');
}

main();

Outputs:

begin
10
65535
100000
10
end