Efficient string building

JavaScript

// arr is a list of characters
let fn = (arr) => {
  let ans = [];
  for (const c of arr) {
    ans.push(c);
  }

  return ans.join("");
};

let fn = (arr) => {
  let ans = "";
  for (const c of arr) {
    ans += c;
  }

  return ans;
};

TypeScript

// arr is a list of characters
let fn = (arr: string[]): string => {
  let ans: string[] = [];
  for (const c of arr) {
    ans.push(c);
  }

  return ans.join("");
};

let fn2 = (arr: string[]): string => {
  let ans: string = "";
  for (const c of arr) {
    ans += c;
  }

  return ans;
};