isWritableStream

Checks if the given argument is a writable stream.

• Check if the value is different from null.
• Use typeof to check if the value is of type object and the pipe property is of type function.
• Additionally check if the typeof the _write and _writableState properties are function and object respectively.

const isWritableStream = val =>
  val !== null &&
  typeof val === 'object' &&
  typeof val.pipe === 'function' &&
  typeof val._write === 'function' &&
  typeof val._writableState === 'object';

const fs = require('fs');

isWritableStream(fs.createWriteStream('test.txt')); // true