AWS Cognito Lambda Pre-signup将在用户在注册之前的验证过程中被调用。它可以执行客户端自定义验证逻辑，并在通过验证后向Cognito发送确认信号。以下是一些在用户注册期间调用Pre-signup的示例Lambda代码：

exports.handler = function(event, context, callback) {
  // Check if email already exists
  if (emailExists(event.request.userAttributes.email)) {
    // Email already exists, reject signup
    callback("Email already exists");
  } else {
    // Email is unique, confirm signup
    event.response.autoConfirmUser = true;
    callback(null, event);
  }
}

function emailExists(email) {
  // Check against users database or perform other email check logic
  return false;
}