merge_user_cognito.js

exports.handler = (event, context, callback) => {

    var AWS = require('aws-sdk');
    var cognitoidentityserviceprovider = new AWS.CognitoIdentityServiceProvider();
    
    console.log(event);
    
   
    if (event.request.userAttributes.hasOwnProperty("email")) {

        var email_filter = {
            "AttributesToGet": null,
            "Filter": "email=\"" + event.request.userAttributes.email + "\"",
            "UserPoolId": event.userPoolId
        };
        
        // get cognito data by email filter, 
        cognitoidentityserviceprovider.listUsers(email_filter, (err, data) => {     
            if (err) console.log(err, err.stack); // an error occurred
            else {
                
                // if no existed user found, sign up normally
                if (!data.Users || data.Users.length == 0) {
                    return callback(null, event);
                }
                
                // we merge and no sign up, but how to sign user in?
                var targetIdentitiesContainer = data.Users[0].Attributes.filter( (attr) => {
                    return attr.Name == 'identities';
                });
                
                var identities = JSON.parse(targetIdentitiesContainer[0].Value);
                
                var targetUserId = identities[0].userId;
                var targetProviderName = identities[0].providerName;

                mergeUserTo({id: targetUserId, provider: targetProviderName} (err, data) =>{
                  return callback(err,event)
                });

            }
        });
    }else{   
      // Return to Amazon Cognito
      // default behavior
      callback(null, event);
    }
};

 //*/
    
function mergeUserTo (target,cb) {

    var params = {
        DestinationUser: { /* required */
            ProviderAttributeValue: target.id, // target.id or target.userName 
            ProviderName: target.provider
          },
        SourceUser: { /* required */
            ProviderAttributeName: 'Cognito_Subject',
            ProviderAttributeValue: event.userName.split('_')[1],
            ProviderName: event.userName.split('_')[0]
        },
        UserPoolId: event.userPoolId
    };

    console.log(params);

    cognitoidentityserviceprovider.adminLinkProviderForUser(params, (err, data) => {
        cb(err,data)
    });
}