random string generator in TypeScript

randomString.ts

export function randomString(n: number, charset?: string): string {
  let res = '';
  let chars =
    charset || 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
  let charLen = chars.length;

  for (var i = 0; i < n; i++) {
    res += chars.charAt(Math.floor(Math.random() * charLen));
  }

  return res;
}