类型 'undefined' 不能分配给类型 'number' .ts(2322)

Question

I'm stuck in learning Typescript language and need some explanations. The problem is that variable named as this.value is never assigned as undefined due isValid function check it. How to make typescript understand it?

export const isValid = (n: any) => n && n > 0 && n < 10;

class Test {
    value: number;
    constructor(value?: number) {
        /*
        Type 'number | undefined' is not assignable to type 'number'.
        Type 'undefined' is not assignable to type 'number'.ts(2322)
        */
        this.value = isValid(value) ? value : -1;
    }
}

Answer 1

As the value in constructor is optional. its type is number | undefined number | undefined , you need to cast it as number when you are assigning it:

 this.value = isValid(value) ? value as number : -1 ;

Answer 2

By default, the type checker does not look at the implementation of called functions, only their signature. Therefore, the typechecker for the constructor does not know that isValid will only return true if n is a number.

You can either inline the code of isValid into the constructor:

constructor(value) {
  this.value = value && value > 0 && value < 10 ? value : -1;
}

or extend the function signature of isValid with a user defined type guard :

export function isValid(n: any): n is number {
  return n && n > 0 && n < 10;
}