Cannot create observable from Observable.bindNodeCallback(fs.readFile) in TypeScript

Question

I am trying to use rxjs 5 to write a Node.js server in TypeScript, but I hit an error when converting fs.readFile to its rxjs form. I expect the following code would work in TypeScript

// This is a JavaScript example from the official documentation. It should
// also work at the TypeScript envrionment.

import * as fs from 'fs';
import { Observable } from 'rxjs';

let readFileAsObservable = Observable.bindNodeCallback(fs.readFile);

// This is the line that throws the error.
let result = readFileAsObservable('./roadNames.txt', 'utf8');

result.subscribe(x => console.log(x), e => console.error(e));

However, my editor reports a TypeScript error when I add the second parameter 'utf-8'

Supplied parameters do not match any signature of call target.

I try to find a guide on how to use the fs.readFile() in rxjs and TypeScript but there isn't much luck.

Answer 1

bindCallback and bindNodeCallback can be tricky with TypeScript, as it all depends upon how TypeScript infers the function parameters.

There is likely a better way, but this is what I do to see exactly what is being inferred: assign the observable to something totally incompatible and look closely at the effected error. For example, this:

const n: number = Observable.bindNodeCallback(fs.readFile);

will effect this error:

Type '(v1: string) => Observable<Buffer>' is not assignable to type 'number'.

So it's obvious that TypeScript is matching the path-only overload of readFile.

In situations like this, I often use an arrow function to specify exactly which overload I want to use. For example, this:

const n: number = Observable.bindNodeCallback((
  path: string,
  encoding: string,
  callback: (error: Error, buffer: Buffer) => void
) => fs.readFile(path, encoding, callback));

will effect this error:

Type '(v1: string, v2: string) => Observable<Buffer>' is not assignable to type 'number'.

So it's now matching the desired overload and the following will work:

let readFileAsObservable = Observable.bindNodeCallback((
  path: string,
  encoding: string,
  callback: (error: Error, buffer: Buffer) => void
) => fs.readFile(path, encoding, callback));

let result = readFileAsObservable('./package.json', 'utf8');
result.subscribe(
  buffer => console.log(buffer.toString()),
  error => console.error(error)
);

Answer 2

To be honest i haven't found a solution, but in order to make it work, i cast it to a function.

(<Function>Rx.Observable.bindNodeCallback(fs.readFile))('./file.txt', 'utf8').subscribe();