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