Argument type number is not assignable to parameter type string | undefined Type number is not assignable to type string

i am bit confused with this warning Argument type number is not assignable to parameter type string | undefined Type number is not assignable to type string

Take the following peace of code :

function createCalculator() {

    let calculator = {
        sum() {
            return this.a + this.b;
        },

        mul() {
            return this.a * this.b;
        },

        read() {
            this.a = +prompt('a?', 0);
            this.b = +prompt('b?', 0);
        }
    };

    calculator.read([1,3,6]);
    console.log( calculator.sum() );
    console.log( calculator.mul() );

}
let calculator;
calculator = createCalculator();

also i have one warning :

Void function return value is used

i want the follow :

the function
createCalculator () returns an object with three methods:

read (arr) accepts a table of numbers and saves it in its field
object.

sum () returns the sum of the table values

mul () returns the product of the table values.