nodejs：检查变量是否为可读流

Question

How i can check if a var is a readable stream in Nodejs?

Example:

function foo(streamobj){

   if(streamobj != readablestream){
       // Error: no writable stream
   }else{
      // So something with streamobj 
   }
}

I tried

if (!(streamobj instanceof stream.Readable)){

But than i get a ReferenceError: stream is not defined

Answer 1

Your problem is definitely that you haven't required stream . But. instanceof is not a good method to check if variable is a readable stream. Consider following cases:

• object can be old-style stream (instance of stream.Stream );
• object can be just emitter with data and end events;
• object can be instance of Readable from external module ( https://github.com/isaacs/readable-stream );

The best way to go is duck typing . Basically, if you are going to pipe stream, check if it has pipe method, if you are going to listen to events, check if stream has on method, etc.

Answer 2

It seems that you forgot to require the stream core module.

var stream = require('stream');

// somewhere in the file
if (!(streamobj instanceof stream.Readable)) {
    // Your logic
}

Answer 3

Make sure you do:

var stream = require('stream');

The function could be something like:

function isReadableStream(obj) {
  return obj instanceof stream.Stream &&
    typeof (obj._read === 'function') &&
    typeof (obj._readableState === 'object');
}

console.log(isReadableStream(fs.createReadStream('car.jpg'))); // true
console.log(isReadableStream({}))); // false
console.log(isReadableStream(''))); // false

Answer 4

Use is-stream package for that:

 var isStream = require('is-stream'); if (isStream.readable(myStream)) { do(); }