app.ts

const aCodeAscii: number = 'a'.charCodeAt(0);
const zCodeAscii: number = 'z'.charCodeAt(0);
const nEnglishLetters: number = zCodeAscii - aCodeAscii + 1;
 
function shiftCipher(str: string, shift: number = 1): string {
    return cipherDecipher(str, shift, mapperCipher);
}
function shiftDecipher(str: string, shift: number = 1) : string {
    return cipherDecipher(str, shift, mapperDecipher);
}
type MapperFunction = (symb: string, shift: number) => string;
function cipherDecipher(str: string, shift: number,
     mapperFun: MapperFunction): string {
        //const arStr: string[] = Array.from(str);
        const arStr: Array<string> = Array.from(str);
        const arRes: Array<string> = arStr.map(symb => {
            let res: string = symb;
            if (symb <= 'z' && symb >= 'a') {
                res = mapperFun(symb, shift);
            }
            return res;
        })
        return arRes.join('');
     }
     function mapperCipher(symb: string, shift: number) : string {
        const actualShift: number = (symb.charCodeAt(0) - aCodeAscii + shift) % nEnglishLetters;
        return String.fromCharCode(aCodeAscii + actualShift);
     }
     function mapperDecipher(symb: string, shift: number): string {
        const actualShift: number = (zCodeAscii - symb.charCodeAt(0) + shift) % nEnglishLetters;
        return String.fromCharCode(zCodeAscii - actualShift);
     }
     type TestObj = {
        str: string,
         shift?: number
     }
     function testCipherDecipher(data: Array<TestObj>,
         testName: string): void{
            console.log(`${"*".repeat(10)}${testName}${"*".repeat(10)}`)
            const funForTest: MapperFunction
             = testName === "cipherTest" ? shiftCipher : shiftDecipher;
data.forEach((obj => console.log(`str=${obj.str}, shift=${obj.shift || 1} => ${funForTest(obj.str, obj.shift)}`))) 
         }
         const dataForCipherTest: Array<TestObj> = [
            {str: "abc"}, {str: "abz", shift: 1000}
         ];
         testCipherDecipher(dataForCipherTest, "cipherTest");
         const dataForDecipherTest: Array<TestObj> = [
            {str: "bcd"}, {str: "mnl", shift: 1000}
         ];
         testCipherDecipher(dataForDecipherTest, "decipherTest");