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function joinStrings(n, f) {
  let s = '';
  for(let i=0; i<n; i++) {
    s += f(i);
  }
  return s;
}

let digits = joinStrings(10, i=> i);
let randomChars = joinStrings(7, i=> String.fromCharCode(Math.floor(Math.random()*26) + 'a'.charCodeAt(0)));

console.log(digits);
console.log(randomChars);