obs7.js

class Observable {

  constructor(functionThatThrowsValues) {
    this._functionThatThrowsValues = functionThatThrowsValues;
  }

  subscribe(next, error, complete) {   
    if (typeof next === "function") {   
      return this._functionThatThrowsValues({
        next,
        error: error || () => {},
        complete: complete || () => {}
      });
    }
    else {
      return this._functionThatThrowsValues(next);
    }
  
  }
  

  map(projectionFunction) { 
      return new Observable(observer => {
        return this.subscribe({
           next(val) { observer.next(projectionFunction(val)) },
           error(e) { observer.error(e) } ,
           complete() { observer.complete() } 
        });
      });
  }
  
  mergeMap(anotherFunctionThatThrowsValues) {
    return new Observable(observer => {
      return this.subscribe({
        next(val) {    
          anotherFunctionThatThrowsValues(val).subscribe({
            next(val) {observer.next(val)},
            error(e) { observer.error(e) } ,
            complete() { observer.complete() } 
          });
        },
        error(e) { observer.error(e) } ,
        complete() { observer.complete() } 
      });
    });
  }
  
  static fromArray(array) {
    return new Observable(observer => {
      array.forEach(val => observer.next(val));
      observer.complete();
    });
  }

  static fromEvent(element, event) {
    return new Observable(observer => {
      const handler = (e) => observer.next(e);
      element.addEventListener(event, handler); 
      return () => {
        element.removeEventListener(event, handler);
      };
    });
  }
  
  static fromPromise(promise) {
    return new Observable(observer => {
      promise.then(val => {
        observer.next(val); observer.complete();
      })
      .catch(e => {
        observer.error(val); observer.complete();
      });
    })
  }
}

Line's 11 and 12 throw "Unexpected token" errors for me. I think if you wrap the fat arrow functions with paren's it should be fine:

error: error || (() => {}),
complete: complete || (() => {})


Great Gist, thank you!