Promise chainの完全理解
https://www.typescriptlang.org/play?#code/MYewdgzgLgBADgJxAWwJYQKYEEYF4YAUAhggOYBcM0CqYpAlJQApJqYA81tpAfHn2AwB3GCxToMBAggwQQAGwBuGADQwA+jIBWGYFHq4eMuUskADACQBvEqQC+geQZAyQxn69GAChQkWInGYAIRg8QltKLjpGUVYJTigaOj5DGEERMTZJaVkFZTVNDB09AyNs0wJLGzI7QFUGFzcPDwB6RsBlBkAxBkAPswdAOwZAGQZALE1AMCVAY2tAdQZAaQZAYwZAfQYPPwysAgByKBIQJfoPYOCAOigACwwwKWMAV3l9Q3mJAKyIc-1N7Zg9w+O7h+LvEwwd+RBSB8LvUmq1OoARBkAZgyza6YRZLLSoJAbLbbV5HE6yT5XGKBIH6dHvM7AwxWb4KX7-QHE-R2epAA