The character 'T' cannot be assigned to the data type 'number'

When working with an optional type argument function RECT(T), I encountered a situation where I need to check if the argument is an instance of date. If it is, I convert it to a number; if not, I use the number directly. However, I keep getting an error that says Type 'T' is not assignable to type 'number'.

rect<T extends Date | number>(x1:T, y1:T,x2?:T, y2?:T) {
    if(x1 instanceof Date) {
      this.opportunityArea.dx = this.returnNumberFunc(x1);
    } else {
      this.opportunityArea.dx = x1; //**TS2322: Type 'T' is not assignable to type 'number'.**
    }
}

Answer №1

The reason it's not functioning properly is due to the fact that the T type is too broad to be fitted into either just a Date or a number. If you utilize a type alias instead of generics, then the function should work correctly.

type DateOrNum = Date | number;

rect(x1: DateOrNum, y1: DateOrNum, x2 ?: DateOrNum, y2 ?: DateOrNum) {
    if (x1 instanceof Date) {
        this.opportunityArea.dx = this.returnNumberFunc(x1);
    } else {
        this.opportunityArea.dx = x1;
    }
}

Answer №2

When working with TypeScript, I often utilize else if(typeof x1 === 'number') and it functions correctly for me

rect<T extends Date | number>(x1:T, y1:T,x2?:T, y2?:T) {
    if(x1 instanceof Date) {
      this.opportunityArea.dx = this.returnNumberFunc(x1);
   } else if(typeof x1 === 'number'){
      this.opportunityArea.dx = x1; //**TS2322: Type 'T' is not assignable to type 'number'.**
    }
}

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