class calc {
    a: number;
    b: number;
    add(a,b) {
        return a + b;
    }
    sub(a, b) {
        return a - b;
    }
    mul(a, b) {
        return a * b;
    }
    divi(a, b) {
        return a / b;
    }
}
 = new calc();

var choice: number;
alert("1.Addition\n2.Subtraction\n3.Multiplication\n4.Division");
alert("Enter your choice");
choice= parseInt(prompt());
    switch(choice) {
        case 1: console.log("Sum of two numbers", opt.add(parseInt(prompt()), parseInt(prompt())));
        break;
        case 2: console.log("Sub of two numbers", opt.sub(parseInt(prompt()), parseInt(prompt())));
        break;
        case 3: console.log("mul of two numbers", opt.mul(parseInt(prompt()), parseInt(prompt())));
        break;
        case 4: console.log("Sum of two numbers", opt.divi(parseInt(prompt()), parseInt(prompt())));
        break;
        default: console.log("Invalid option");
    }