Getting data from a stream

Question

Greetings. I'm trying to get data from the stream.

process.stdin.setEncoding('utf8'); process.stdin.on('readable', () => { let chunk = String(process.stdin.read()); console.log(chunk); });

I'm coming back

 Input: 10 2 Your output: 10 2 null

How to get rid of null?

Accepted Answer · 2017-03-07T18:31:48

Alternatively, check that process.stdin.read() returned something useful (not null ).

Naturally, you need to check before the cast to the line, and not after. Like that:

 process.stdin.on('readable', () => { var chunk = process.stdin.read(); if (!chunk) { return; } chunk = "" + chunk; console.log(chunk); });

Qwertiy ♦

76.6k 17 golden marks 74 silver marks 203 bronze marks

@DmitriySimushev, well, so I output to the console that this is true. I did not understand what was wrong. - Qwertiy ♦
@DmitriySimushev, can be more detailed comments. As far as I understand, process.stdin.read() returned null (itself null, not a string, not a buffer, but null), it ran it without checking to the string "null" and outputted it. I am wrong? - Qwertiy ♦
@DmitriySimushev, so you need to check before the cast, and not after. I wrote that you need to compare the process.stdin.read() with null , and not the chunk with "null" . - Qwertiy ♦
@DmitriySimushev, clarified the answer. - Qwertiy ♦
@DmitriySimushev, minus something you remove? - Qwertiy ♦

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Answer 2 · 2017-03-07T18:40:27

Why is this happening?

The readable event is thrown not only when a new piece of data is available, but also when the end of the stream is reached:

Here is what is said in the official documentation :

This is a "readable" event.

Further, if you try to read from the stream after the end of the stream has been reached, the stream.read method will return null .

Let me quote the documentation again:

Note: Calling stream.read([size]) after the 'end' event has been returned. No runtime error will be raised.

In other words, a null return is the regular behavior of a readable stream.

What to do?

There are several options here:

The stream.read method returns null only one case. You can explicitly check the result of stream.read for this value:

 process.stdin.on('readable', () => { let chunk = process.stdin.read(); if (null !== chunk) { console.log(chunk.toString()); } });

You can use the stream in a different ( flowing ) mode and receive data on the data event:
```
 process.stdin.on('data', (chunk) => { console.log(chunk.toString()); }); 
```

Hmm .. and in the second case, provided that he used setEncoding wouldn't the chunk be a string right away?
On the other hand, in the answer I do not use setEncoding , so conversion to a string is necessary

2 answers 2