要使用AWS Cognito前端RBAC的自定义角色，您可以通过以下步骤实现：

1. 创建自定义角色：

   const customRole = {
     roleArn: 'arn:aws:iam::123456789012:role/CustomRole',
     roleSessionName: 'CustomRoleSession'
   };
   

2. 创建AWS Cognito身份提供者：

   const cognitoIdentityProvider = new AWS.CognitoIdentityProvider();
   

3. 获取Cognito用户的身份ID：

   const getCognitoIdentityId = async () => {
     const cognitoIdentity = new AWS.CognitoIdentity();
     const { IdentityId } = await cognitoIdentity.getId({
       IdentityPoolId: 'your-identity-pool-id'
     }).promise();
     return IdentityId;
   };
   

4. 获取Cognito用户的身份令牌：

   const getCognitoToken = async () => {
     const { IdentityId } = await getCognitoIdentityId();
     const GetCredentialsForIdentityResponse = await cognitoIdentityProvider.getCredentialsForIdentity({
       IdentityId,
       Logins: {
         'cognito-idp..amazonaws.com/': 'your-cognito-user-token'
       }
     }).promise();
     return GetCredentialsForIdentityResponse.Credentials;
   };
   

5. 使用自定义角色获取临时凭证：

   const getCustomRoleCredentials = async () => {
     const { AccessKeyId, SecretKey, SessionToken } = await getCognitoToken();
     const sts = new AWS.STS({
       accessKeyId: AccessKeyId,
       secretAccessKey: SecretKey,
       sessionToken: SessionToken
     });
     const assumeRoleResponse = await sts.assumeRole({
       RoleArn: customRole.roleArn,
       RoleSessionName: customRole.roleSessionName
     }).promise();
     return assumeRoleResponse.Credentials;
   };
   

6. 调用getCustomRoleCredentials函数获取自定义角色的临时凭证并使用它进行后续操作：

   const customRoleCredentials = await getCustomRoleCredentials();
   const customRoleSTSClient = new AWS.STS({
     accessKeyId: customRoleCredentials.AccessKeyId,
     secretAccessKey: customRoleCredentials.SecretAccessKey,
     sessionToken: customRoleCredentials.SessionToken
   });
   
   // 在此处使用customRoleSTSClient进行后续操作，例如调用AWS服务API
   

请确保替换示例代码中的占位符（如your-identity-pool-id，，等）为您的实际值。此外，还需要安装AWS SDK并设置AWS凭证信息（密钥和访问密钥）。