А тебе просто поменять на obj[], вместо [obj]
https://www.typescriptlang.org/play?#code/PTAECcFMEcFdIM4BdQBMCGT0CgDGB7AO2TU3VAF5QBvbUe0WBScBALlAG1rD0BbSBwCMAXwA0NXgOHjJ-QaFEBdbCOwhs2AJaEkLAGbpckUAEkACuHwAHBDTqgjBWLvYP6tBgykLCsPgBGLADc7qAiXEqgAD6gfgA28aFqeEQk1la2HBaZdlSeDE74LkjscbCJDiKhDhk2CAB0RSV5pFgNTCwIwUA