index.ts

import { question } from "readline-sync";

type Operator = '+' | '-' | '*' | '/';

function main(): void {
    const firstStr: string = question('Enter first number:\n');
    const operator: string = question('Enter Operator:\n');
    const secondStr: string = question('Enter second number\n');

    const validInput: boolean = isNumber(firstStr) && isOperator(operator) && isNumber(secondStr);

    if (validInput) {
        // console.log('is valid.')
        const firstNum: number = parseInt(firstStr)
        const secondNum: number = parseInt(secondStr)
        const result = calculate(firstNum, operator as Operator, secondNum)
        console.log(result)
    } else {
        console.log('\ninvalid input\n')
        main()
    }

}

function calculate(firstNum: number, operator: Operator, secondNum: number) {

    switch (operator) {
        case '+':
            return firstNum + secondNum;
        case '-':
            return firstNum - secondNum;
        case '*':
            return firstNum * secondNum;
        case '/':
            return firstNum / secondNum;
    }

}

function isOperator(operator: string): boolean {

    switch (operator) {
        case '+':
        case '-':
        case '*':
        case '/':
            return true;
        default:
            return false

    }
}

function isNumber(str: string): boolean {
    const maybeNum = parseInt(str);
    const isNum: boolean = !isNaN(maybeNum);
    return isNum;
}
main()