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function randomString(len) {
  var result = '';
  var characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
  var charLen = characters.length;
  for (var i = 0; i < len; i++) {
    result += characters.charAt(Math.floor(Math.random() * charLen));
  }
  return result;
}

randomString(8); // "UT4wgce4"

randomString(10); // "0J5r5r5rDx"

// 生成a-z的隨機字符串
function randomLowerCaseString(len) {
  var result = '';
  var characters = 'abcdefghijklmnopqrstuvwxyz';
  var charLen = characters.length;
  for (var i = 0; i < len; i++) {
    result += characters.charAt(Math.floor(Math.random() * charLen));
  }
  return result;
}

// 生成A-Z的隨機字符串
function randomUpperCaseString(len) {
  var result = '';
  var characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
  var charLen = characters.length;
  for (var i = 0; i < len; i++) {
    result += characters.charAt(Math.floor(Math.random() * charLen));
  }
  return result;
}

// 生成0-9的隨機字符串
function randomNumberString(len) {
  var result = '';
  var characters = '0123456789';
  var charLen = characters.length;
  for (var i = 0; i < len; i++) {
    result += characters.charAt(Math.floor(Math.random() * charLen));
  }
  return result;
}

// 生成a-z、A-Z、0-9的隨機字符串
function randomAlphaNumericString(len) {
  var result = '';
  var characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
  var charLen = characters.length;
  for (var i = 0; i < len; i++) {
    result += characters.charAt(Math.floor(Math.random() * charLen));
  }
  return result;
}

randomLowerCaseString(6); // "opsmgo"
randomUpperCaseString(6); // "YHSBCZ"
randomNumberString(6); // "837492"
randomAlphaNumericString(8); // "lG7psKb5"