How to create an Expo app with serverless

In this example we will look at how to use Expo with a serverless API to create a simple click counter app. We’ll be using the Serverless Stack Framework (SST).

Requirements

• Node.js >= 10.15.1
• We’ll be using Node.js (or ES) in this example but you can also use TypeScript
• An AWS account with the AWS CLI configured locally

Create an SST app

Let’s start by creating an SST app.

$ npx create-serverless-stack@latest expo-app
$ cd expo-app

By default our app will be deployed to the us-east-1 AWS region. This can be changed in the sst.json in your project root.

{
  "name": "expo-app",
  "region": "us-east-1",
  "main": "stacks/index.js"
}

Project layout

An SST app is made up of a couple of parts.

1. stacks/ — App Infrastructure

   The code that describes the infrastructure of your serverless app is placed in the stacks/ directory of your project. SST uses AWS CDK, to create the infrastructure.

2. src/ — App Code

   The code that’s run when your API is invoked is placed in the src/ directory of your project.

3. frontend/ — Expo App

   The code for our frontend Expo app.

Create our infrastructure

Our app is made up of a simple API and a Expo app. The API will be talking to a database to store the number of clicks. We’ll start by creating the database.

Adding the table

We’ll be using Amazon DynamoDB; a reliable and highly-performant NoSQL database that can be configured as a true serverless database. Meaning that it’ll scale up and down automatically. And you won’t get charged if you are not using it.

Replace the stacks/MyStack.js with the following.

import * as sst from "@serverless-stack/resources";

export default class MyStack extends sst.Stack {
  constructor(scope, id, props) {
    super(scope, id, props);

    // Create the table
    const table = new sst.Table(this, "Counter", {
      fields: {
        counter: sst.TableFieldType.STRING,
      },
      primaryIndex: { partitionKey: "counter" },
    });
  }
}

This creates a serverless DynamoDB table using the SST Table construct. It has a primary key called counter. Our table is going to look something like this:

Creating our API

Now let’s add the API.

Add this below the sst.Table definition in stacks/MyStack.js.

// Create the HTTP API
const api = new sst.Api(this, "Api", {
  defaultFunctionProps: {
    // Pass in the table name to our API
    environment: {
      tableName: table.dynamodbTable.tableName,
    },
  },
  routes: {
    "POST /": "src/lambda.main",
  },
});

// Allow the API to access the table
api.attachPermissions([table]);

// Show the API endpoint in the output
this.addOutputs({
  ApiEndpoint: api.url,
});

We are using the SST Api construct to create our API. It simply has one endpoint (the root). When we make a POST request to this endpoint the Lambda function called main in src/lambda.js will get invoked.

We also pass in the name of our DynamoDB table to our API as an environment variable called tableName. And we allow our API to access (read and write) the table instance we just created.

Reading from our table

Our API is powered by a Lambda function. In the function we’ll read from our DynamoDB table.

Replace src/lambda.js with the following.

import AWS from "aws-sdk";

const dynamoDb = new AWS.DynamoDB.DocumentClient();

export async function main() {
  const getParams = {
    // Get the table name from the environment variable
    TableName: process.env.tableName,
    // Get the row where the counter is called "clicks"
    Key: {
      counter: "clicks",
    },
  };
  const results = await dynamoDb.get(getParams).promise();

  // If there is a row, then get the value of the
  // column called "tally"
  let count = results.Item ? results.Item.tally : 0;

  return {
    statusCode: 200,
    body: count,
  };
}

We make a get call to our DynamoDB table and get the value of a row where the counter column has the value clicks. Since we haven’t written to this column yet, we are going to just return 0.

Let’s install the aws-sdk.

$ npm install aws-sdk

And let’s test what we have so far.

Starting your dev environment

SST features a Live Lambda Development environment that allows you to work on your serverless apps live.

$ npx sst start

The first time you run this command it’ll take a couple of minutes to deploy your app and a debug stack to power the Live Lambda Development environment.

===============
 Deploying app
===============

Preparing your SST app
Transpiling source
Linting source
Deploying stacks
dev-expo-app-my-stack: deploying...

 ✅  dev-expo-app-my-stack


Stack dev-expo-app-my-stack
  Status: deployed
  Outputs:
    ApiEndpoint: https://sez1p3dsia.execute-api.ap-south-1.amazonaws.com

The ApiEndpoint is the API we just created.

Let’s test our endpoint. Run the following in your terminal.

$ curl -X POST https://sez1p3dsia.execute-api.ap-south-1.amazonaws.com

You should see a 0 printed out.

Setting up our Expo app

We are now ready to use the API we just created. Let’s use Expo CLI to setup our Expo app.

Run the following in the project root and create a blank project.

$ npm install -g expo-cli
$ expo init frontend
$ cd frontend

This sets up our Expo app in the frontend/ directory.

We also need to load the environment variables from our SST app. To do this, we’ll be using the babel-plugin-inline-dotenv package.

Install the babel-plugin-inline-dotenv package by running the following in the frontend/ directory.

$ npm install babel-plugin-inline-dotenv

We need to update our script to use this package in babel.config.js.

Update your babel.config.js like below.

module.exports = function (api) {
  api.cache(true);
  return {
    presets: ["babel-preset-expo"],
    plugins: ["inline-dotenv"],
  };
};

Create a .env file inside frontend/ and create two variables to hold dev and prod API endpoints and replace DEV_API_URL with the deployed URL from the steps above.

DEV_API_URL=https://sez1p3dsia.execute-api.us-east-1.amazonaws.com
PROD_API_URL=<TO_BE_ADDED_LATER>

We’ll add the PROD_API_URL later in this example.

Let’s start our Expo development environment.

In the frontend/ directory run the following for the iOS emulator.

$ expo start --ios

Or run this for the Android emulator.

$ expo start --android

This will open up an emulator and load your app.

Add the click button

We are now ready to add the UI for our app and connect it to our serverless API.

Replace frontend/App.js with.

/* eslint-disable no-undef */
import { StatusBar } from "expo-status-bar";
import React, { useState } from "react";
import { StyleSheet, Text, TouchableOpacity, View } from "react-native";

export default function App() {
  const [count, setCount] = useState(0);

  const API_URL = __DEV__ ? process.env.DEV_API_URL : process.env.PROD_API_URL;

  function onClick() {
    fetch(API_URL, {
      method: "POST",
    })
      .then((response) => response.text())
      .then(setCount);
  }

  return (
    <View style={styles.container}>
      <StatusBar style="auto" />
      <Text>You clicked me {count} times.</Text>
      <TouchableOpacity style={styles.btn} onPress={onClick}>
        <Text>Click me!</Text>
      </TouchableOpacity>
    </View>
  );
}

Here we are adding a simple button that when clicked, makes a request to our API. We are getting the API endpoint from the environment variable, process.env.API_URL.

The response from our API is then stored in our app’s state. We use it to display the count of the number of times the button has been clicked.

Let’s add some styles.

Add a StyleSheet in your App.js.

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: "#fff",
    alignItems: "center",
    justifyContent: "center",
  },
  btn: {
    backgroundColor: "lightblue",
    padding: 10,
    margin: 10,
    borderRadius: 5,
  },
});

Now if you head over to your emulator, your Expo app should look something like this.

Of course if you click on the button multiple times, the count doesn’t change. That’s because we are not updating the count in our API. We’ll do that next.

Making changes

Let’s update our table with the clicks.

Add this above the return statement in src/lambda.js.

const putParams = {
  TableName: process.env.tableName,
  Key: {
    counter: "clicks",
  },
  // Update the "tally" column
  UpdateExpression: "SET tally = :count",
  ExpressionAttributeValues: {
    // Increase the count
    ":count": ++count,
  },
};
await dynamoDb.update(putParams).promise();

Here we are updating the clicks row’s tally column with the increased count.

And if you head over to your emulator and click the button again, you should see the count increase!

Deploying to prod

To wrap things up we’ll deploy our app to prod.

$ npx sst deploy --stage prod

This allows us to separate our environments, so when we are working locally it doesn’t break the app for our users.

Once deployed, you should see something like this.

 ✅  prod-expo-app-my-stack


Stack prod-expo-app-my-stack
  Status: deployed
  Outputs:
    ApiEndpoint: https://k40qchmtvf.execute-api.ap-south-1.amazonaws.com

Add the above endpoint to the .env file in frontend/.env as the production API endpoint

DEV_API_URL=https://hfv2gyuwdh.execute-api.us-east-1.amazonaws.com
PROD_API_URL=https://k40qchmtvf.execute-api.us-east-1.amazonaws.com

Now we are ready to ship our app!

Cleaning up

Finally, you can remove the resources created in this example using the following commands.

$ npx sst remove
$ npx sst remove --stage prod

Conclusion

And that’s it! We’ve got a completely serverless click counter Expo app. A local development environment, to test and make changes. And it’s deployed to production as well, so you can share it with your users. Check out the repo below for the code we used in this example. And leave a comment if you have any questions!