AWS Lambda (use lambda proxy) for registering FCM refresh token to AWS SNS

lambda.js

const AWS = require('aws-sdk');
const SnsClient = new AWS.SNS();

const createEndpoint = (platformAppArn, token, callback) => {

    const createParams = {
        PlatformApplicationArn: platformAppArn,
        Token: token
    };

    SnsClient.createPlatformEndpoint(createParams, (err, data) => {
        
        if(err) {
            return callback(err);
        }
        else {
            const endpointArn = data.EndpointArn;
            
            const res = {
                statusCode: 200,
                headers: {},
                body: JSON.stringify({ app_token : endpointArn })
            };

            return callback(null, res);
        }
    });
};

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

    // Application ARN is passed as ENVironnement variable
    const appArn = process.env.SNS_APP_ARN;

    // Get body from request
    let body = event["body"];

    // parse local data
    if(typeof body === "string") {
        body = JSON.parse(body);
    }

    // Retrieve useful data
    const refreshToken = body['refresh_token'];
    let appToken = body['app_token'];

    // easier
    const endpointArn = appToken;

    // First time registration, create endpoint
    if(!appToken) {
        createEndpoint(appArn, refreshToken, callback);
    }
    // Already created
    else {

        // Get attributes
        SnsClient.getEndpointAttributes({ EndpointArn: endpointArn }, (err, data) => {

            // not found or issue, assume we need to recreate
            if(err) {
                createEndpoint(appArn, refreshToken, callback);
            }
            // endpoint found
            else {

                const update = data.Attributes.Token !== refreshToken || data.Attributes.Enabled !== true;

                // need update
                if(update) {
                    const updateParams = {
                        EndpointArn: endpointArn,
                        Attributes: {
                            Token: refreshToken,
                            Enabled: "true"
                        }
                    };

                    SnsClient.setEndpointAttributes(updateParams, (err, data) => {

                        if(err) return callback(err);
                        else {

                            const res = {
                                statusCode: 200,
                                headers: {},
                                body: JSON.stringify({})
                            };

                            return callback(null, res);
                        }
                    });
                }
                // no update needed, we can go
                else {

                    const res = {
                        statusCode: 200,
                        headers: {},
                        body: JSON.stringify({})
                    };

                    return callback(null, res);
                }
            }
        });
    }
};