Unlike mergeMap and switchMap, concatMap focuses on when "inner" subscriptions "complete" by using a "buffer". Each time concatMap receives a value, it adds each value to a "buffer", waits for previous "inner" subscription to complete, then invokes next with the next value from the "buffer".

 

class MyConcatMapSubscriber extends Subscriber {  innerSubscription;  buffer = [];  constructor(sub, fn) {    super(sub);    this.fn = fn;  }  _next(value) {    const { isStopped } = this.innerSubscription || {      isStopped: true    };    if (!isStopped) {      this.buffer = [...this.buffer, value];    } else {      const o$ = this.fn(value);      this.innerSubscription = o$.subscribe({        next: value => {          this.destination.next(value);        },        complete: () => {          if (this.buffer.length) {            const { first, ...rest } = this.buffer;            this.buffer = rest;            this._next(first);          }        }      });    }  }}