问“函数断言foo是类型”

EN

class A {
    constructor() {}
}

class B extends A {
    value: number;

    constructor(value: number) {
        super();
        this.value = value;
    }
}


const values: A[] = [new A(), new A(), new B(123)];
const oneOfThem = values[2];

validateB(oneOfThem);

console.log(oneOfThem.value);


function validateB(item: A): asserts item is B {
    if (!(item instanceof B)) {
        throw new Error('wrong type!');
    }
}

function validateBWrapper(item: A) {
    return validateB(item);
}