Rx.Observable.fromCallback(func, [scheduler], [context], [selector])

Converts a callback function to an observable sequence.


  1. func (Function): Function with a callback as the last parameter to convert to an Observable sequence.
  2. [scheduler=Rx.Scheduler.timeout] (Scheduler): Scheduler to run the function on. If not specified, defaults to Rx.Scheduler.timeout.
  3. [context] (Any): The context for the func parameter to be executed. If not specified, defaults to undefined.
  4. [selector] (Function): A selector which takes the arguments from the callback to produce a single item to yield on next.


(Function): A function, when executed with the required parameters minus the callback, produces an Observable sequence with a single value of the arguments to the callback as an array if no selector given, else the object created by the selector function.


var fs = require('fs'),
    Rx = require('rx');

// Wrap fs.exists
var exists = Rx.Observable.fromCallback(fs.exists);

// Check if file.txt exists
var source = exists('file.txt');

var subscription = source.subscribe(
    function (x) {
        console.log('Next: ' + result);
    function (err) {
        console.log('Error: ' + err);   
    function () {

// => Next: true
// => Completed