Observable.tagError: Basically to help manually debug JavaScript applications by using locator tags. The locator tags are prepended to the error as it travel to the consumer.

example1.js

Rx.Observable
  .concat(
    [
      Rx.Observable.return('{ "value": 0 }'),
      Rx.Observable.return('{ "value": 5 }'),
      Rx.Observable.return("{")
    ]
  )
  .tagError('Source')
  .tagError('ParseJSON', function (source) {
    return source.map(function (s) { return JSON.parse(s); });
  })
  .map(function (obj) { return 2 / obj.value; })
  .subscribe(
    console.log.bind(console, '     onNext ->'),
    console.log.bind(console, '    onError ->'),
    console.log.bind(console, 'onCompleted ->')
  );

example2.js

Rx.Observable
  .concat(
    [
      Rx.Observable.return('{ "value": 0 }'),
      Rx.Observable.return('{ "value": 5 }'),
      Rx.Observable.return("{")
    ]
  )
  .tagError('Source')
  .map(function (s) { return JSON.parse(s); })
  .tagError('ParseJSON')
  .map(function (obj) { return 2 / obj.value; })
  .subscribe(
    console.log.bind(console, '     onNext ->'),
    console.log.bind(console, '    onError ->'),
    console.log.bind(console, 'onCompleted ->')
  );

tagError.js

function Error (message) {
  this.message = message;
  this.tags = [];
}

Rx.Observable.prototype.tagError = function (tag, selector) {
  return selector
    ? selector(this).tagError(tag)
    : this.catchException(function (error) {
        var isError = error instanceof Error;
        if (!isError) {
          error = new Error(error);
        }
        error.tags.unshift(tag);
        return Rx.Observable.throw(error);
      });
}