@loopback/docs/site/exposing-graphql-apis.md

Documentation for LoopBack 4

---
lang: en
title: 'Exposing GraphQL APIs'
keywords: LoopBack 4.0, LoopBack 4, GraphQL
sidebar: lb4_sidebar
permalink: /doc/en/lb4/exposing-graphql-apis.html
---

## Overview

The [OASGraph module](https://www.npmjs.com/package/oasgraph) creates a GraphQL
wrapper for existing REST APIs which are described by the OpenAPI specification.
This tutorial shows how to expose GraphQL APIs in an existing LoopBack
application.

### Prerequisite

Make sure you have a running LoopBack 4 application. In this tutorial, we'll use
the `todo` example. You can create this application by running the command
below:

```sh
lb4 example todo
```

### Install OASGraph and Required Dependencies

From your LoopBack application, run the following command to install OASGraph
and the required dependencies:

```sh
npm i --save oasgraph-cli
```

### Start the GraphQL Server

Make sure your LoopBack application is running by going to
`http://localhost:3000/openapi.json`. If not, you can start it by running the
`npm start` command.

Now we will use the oasgraph CLI to set up a GraphQL HTTP Server backed by
express on port 3001. Specifying the OpenAPI spec generated by the
todo-application as the parameter, start up the server by running the following
command:

```sh
npx oasgraph http://localhost:3000/openapi.json
```

_Haven't heard about `npx` yet? It's a cool helper provided by `npm` and
available out of the box since Node.js 8.x. Learn more in their announcement
blog post:
[Introducing npx: an npm package runner](https://medium.com/@maybekatz/introducing-npx-an-npm-package-runner-55f7d4bd282b)_

That’s it! You’re now ready to try out some tests and requests in the browser at
http://localhost:3001/graphql.

{% include note.html content="
We are looking into ways how to expose the GraphQL endpoint alongside the main REST API,
on the same HTTP host and port. See
[issue #1905](https://github.com/strongloop/loopback-next/issues/1905).
" %}

### Try Out the GraphQL APIs

Here are some examples of the `query` and `mutation` calls:

1. To get all the to-do instances, run this query command:

```
 query{
   todos {
     id
     title
     desc
   }
 }
```

The expected output looks like this:

```json
{ "data": { "todos": [
      { "id": 1, "title": "Take over the galaxy", "desc": "MWAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHA" },
      { "id": 2, "title": "destroy alderaan", "desc": "Make sure there are no survivors left!" },
      {"id": 3, "title": "terrorize senate", "desc": "Tell them they're getting a budget
  cut." },
      { "id": 4, "title": "crush rebel scum", "desc": "Every.Last.One." }
] } }
```

2. Create a to-do instance and retrieve its ID and title in the response object
   using the following mutation command:

```
 mutation {
   todoControllerCreateTodo(todoInput: {
     title: "Take over the universe"
   }) {
     id
     title
   }
 }
```

The expected output looks like this:

```json
{
  "data": {
    "todoControllerCreateTodo": {
      "id": 5,
      "title": "Take over the universe"
    }
  }
}
```