Why createReadStream instance of Buffer showing false?

[mlhazan]

I was practicing stream. The code below works fine put put me in a tough situation to figure out 2 things:

1. Why I am getting this output twice :

   typeof d :object
   false

2. why d instanceof Buffer is false. if False then how streaming is working in the else if condition?

var fs = require('fs');

var contents;

var rs = fs.createReadStream("simple_stream.js");

rs.on('readable', () => {
    var str;
    var d = rs.read();
    console.log("typeof d :"+typeof d);
   console.log("d instanceof Buffer: "+d instanceof Buffer)
    if (d) {
        if (typeof d == 'string') {
            str = d;
        } else if (typeof d == 'object' && d instanceof Buffer) {
            str = d.toString('utf8')
        }
        if (str) {
            if (!contents)
                contents = d;
            else contents += str;
        }
    }
});

rs.on('end', () => {
    console.log("read in the file contents");
    console.log(contents.toString('utf8'))
})

[m_hutley]

@mlhazan

Why I am getting this output twice

To quote the node.js documentation:
“The ‘readable’ event will also be emitted once the end of the stream data has been reached but before the ‘end’ event is emitted.”

@mlhazan

why d instanceof Buffer is false. if False then how streaming is working in the else if condition?

This is the part i’m not sure on. The second time through, d should be null…
what happens if you tell it to flat out console.log(d) ?

[mlhazan]

@m_hutley

“The ‘readable’ event will also be emitted once the end of the stream data has been reached but before the ‘end’ event is emitted.”

console.log(d) : d → null
I believe d is flushed. But that is understandable . Do you know any one who can answer my second question?

[m_hutley]

What does it output the other time it runs (the one with what you expect to be a Buffer)?

[mlhazan]

Actually the following is true even though d instanceof Buffer is false. May be somehow it becomes true when goes to the if else. It’s an assumption

else if (typeof d == 'object' && d instanceof Buffer) {
            str = d.toString('utf8')

[m_hutley]

first impulse: Theory: instanceof has a lower priority than &&.
Try else if ((typeof d == 'object') && (d instanceof Buffer)) {