Very simple encoder that supports UTF-16 characters

encoder.ts

class Encoder {
    public static encode(str: string, pass: string): string {
        let res = '';
        let j = 0, jlen = pass.length;
        str = pass + str;

        for (let i = 0, ilen = str.length; i < ilen; i++) {
            let charCode = str.charCodeAt(i),
                passCode = pass.charCodeAt(j),
                sumCode  = charCode + passCode;

            let code = sumCode > 0xffff ? sumCode - 0xffff : sumCode;
            res += String.fromCharCode(code);
            j = j + 1 >= jlen ? 0 : j + 1;
        }
        
        return res;
    }

    public static decode(str: string, pass: string): string|null {
        let res = '';
        let j = 0, jlen = pass.length;

        for (let i = 0, ilen = str.length; i < ilen; i++) {
            let charCode = str.charCodeAt(i),
                passCode = pass.charCodeAt(j),
                diffCode  = charCode - passCode;

            let code = diffCode < 0 ? diffCode + 0xffff : diffCode;
            res += String.fromCharCode(code);
            j = j + 1 >= jlen ? 0 : j + 1;
        }

        return res.substr(0, jlen) === pass ? res.substr(jlen) : null;
    }
}


let str = 'y%$E' + String.fromCodePoint(0x10000);
let pass = 'hello';
let enc = Encoder.encode(str, pass);
let dec = Encoder.decode(enc, pass);
console.log(str, pass, enc, dec, str === dec);