## Generating the REST API with init()

After `generateGqlQueryFiles()` is executed once, `init()` can be executed to create REST routes at runtime.

In this example, we'll use [Apollo Link](https://www.apollographql.com/docs/link/links/http/) to connect via HTTP to our GraphQL server which runs on `localhost` (however this could be any type GraphQL server, running locally or remote.)

Using Apollo Link with `fetch` is the simplest way to connect GraphQL2REST to an existing GraphQL server.


#### For example:
```js
const path = require('path');
const { schema } = require('./myGraphQLSchema.js'); // a GraphQLSchema object
const GraphQL2REST = require('graphql2rest');


const { execute, makePromise } = require('apollo-link');
const { createHttpLink } = require('apollo-link-http');
const fetch = require('node-fetch');

const express = require('express');
const app = express();

const gqlServerUri = 'http://localhost:3000/api/graphql'; // our existing GraphQL server

const link = createHttpLink({ uri: gqlServerUri, fetch });

/* GraphQL2REST execute function using apollo-link. Invokes GraphQL operation against gqlServerUri via node-fetch */
const executeGqlLink = (operation) => {
      return makePromise(execute(link, operation));
};

const GQL_FILES_FOLDER = path.resolve(__dirname,'./gqlFilesFolder'); // folder previously generated by generateGqlQueryFiles()

const gql2restOptions = {
	apiPrefix: '/api', //sets the API base path url
	manifestFile: path.resolve(__dirname,'./api-manifest.json'), //pathname of manifest file. Default is ./manifest.json
	gqlGeneratorOutputFolder: GQL_FILES_FOLDER  //.gql files folder
};

const restRouter = GraphQL2REST.init(schema, executeGqlLink, gql2restOptions);

// restRouter now has our REST API attached
app.use('/', restRouter);


```

<br>

Now the following REST API is mounted and active (based on the [manifest file described earlier](manifest-example.json)):

> GET /api/users

> GET /api/users/{userId}

> POST /api/users

> PATCH /api/users/{userId}

> DELETE /api/users/{userId}

<br>

### The init() function


`init()` takes two mandatory parameters: your GraphQL **schema** and the GraphQL server ***execute function*** (whatever your specific GraphQL server implementation provides, or an Apollo Link function).

```ts
GraphQL2REST.init(
      schema: GraphQLSchema,
      executeFn: Function,

      options?: Object,
      formatErrorFn?: Function,
      formatDataFn?: Function,
      expressRouter?: Function)
```
`schema` is a GraphQLSchema object. You can generate this object using `buildSchema()` function from the `graphql` package.

`executeFn` is a function that, similarly to Apollo Link, accepts a single object with `{ query, variables, context, operationName }` arguments.

 If you want to use another kind of GraphQL execution function, which receives different arguments, you can wrap that GraphQL 'execute' function with `executeFn`, and map the arguments so that the final execution function receives the right arguments and values (see the example under */examples*).

 <br>

`options` can be used to define specific settings (see below). If undefined, default values will be used.


`formatErrorFn` is an optional function to custom format GraphQL error responses. It receives `(response, statusCode)` arguments where `response` is the original response from GraphQL and `statusCode` is the final HTTP status code. It should return the formatted response, to be sent by REST API to the user.

`formatDataFn` is an optional function to custom format non-error GraphQL responses (data). It receives a `response` argument which is the original response from GraphQL.

If not provided, default behavior is to strip the encapsulating `'data:'` property and operation name, and omit the `'errors'` array from successful responses.

`expressRouter` is an express.Router() instance to attach new routes to (if not provided, a new Express instance will be returned).

<br>

#### The options object:
For any fields not specified in the *options* object, or if *options* is not provided to init(), values from the *defaults.json* file will be used.

These are all optional settings that can be defined in `options`:
```js
const gql2restOptions  = {
	apiPrefix: '/api/v2', //sets the API base path url
	manifestFile: './api-v2-manifest.json', //pathname of manifest file
	gqlGeneratorOutputFolder: './gqls', //.gql files folder (generated by generateGqlQueryFiles())
	middlewaresFile:  './middlewares.js', //optional middlewares module for modifying requests
	filterFieldName: 'fields', //global query parameter name for filtering (default is 'fields')
	graphqlErrorCodeObjPath:  'errors[0].extensions.code', //property name for GraphQL error code for error mapping
	logger: myCustomLogger //optional Winston-based logger function
};

const expressRouter = GraphQL2REST.init(schema, execute, gql2restOptions);
```
Use ``path.resolve(__dirname, <PATH>)`` for relative paths.

All fields in `options` are optional, but init() will not be able to run without a valid manifest file and gqlGeneratorOutputFolder previously populated by `generateGqlQueryFiles()`.


<br>

---

Learn more about:
- [Filtering and shaping the responses on the client side](Client%20filters.md)
- [Customizing and formatting response format](Formatting%20responses.md)
- [Using apollo-link to work with a remote GraphQL server](Using%20remote%20GraphQL%20server.md)


<br>

[Back to [the tutorial](https://github.com/sisense/graphql2rest#tutorial)]