如何允许返回类型

Question

考虑以下代码：

type deviceStatusType = string | boolean | number | undefined;

class A {
 protected constructor() {
        this.status = {
        "a": 1,
        "b": "b"
        }
    }

    getDeviceStatus<T extends deviceStatusType>(arg: deviceStatusType,  code: string): T {
        const status = this.status[code];

        let actualType = typeof status;
        if (actualType !== arg){
            throw new Error(`mode should be a ${arg} but is: ${typeof(status)}`)
        }

        return status;
    }
}

我得到了错误

TS2322: Type 'string | number | boolean' is not assignable to type 'T'.   'string | number | boolean' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'deviceStatusType'.     Type 'string' is not assignable to type 'T'.       'string' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'deviceStatusType'.

on返回状态-但我真的希望能够将类型检查移动到此函数中，因为这减少了我必须将类型检查到一个位置的次数。

Answer 1

由于您已经完成了显式类型检查，您可以将状态声明为任何可以分配给T的返回值的代码。如果你愿意，你可以稍微改进一下

class A {
  getDeviceStatus<T extends deviceStatusType>(arg: T, code: string): T {
    const status = this.status[code];
    if (isType(status, arg)) {
        return status;
    }
    throw new Error(`mode should be a ${arg} but is: ${typeof(status)}`);
  }
}

function isType<T extends deviceStatusType>(actual: any, arg: T): actual is T {
  return typeof actual === arg;
}