JSON Server

Aug 16, 2017

Hello, Today I’m going to talk about json-server, what is it? why use it? and especially how to use it?

json-server is a cool npm module, that provides an Express server that serves a JSON API.

Why use it?

Let's say you are starting to work on your awesome (Javascript, PHP, IOS, Android, … whatever) app, and you want to consume remote data of some, not finished yet, API. The first step you take consists in setting up some mocks, either hard coded in some constant (please, don't do this), or using a static JSON file, which will be a pain in the neck. Why? You're a good developer who likes to do the right thing. You want to test your app against data updates. You want to perform HTTP requests (like GET, POST …etc.), and you want to persist your updates. Unfortunately, you can't do all of this using a static file, you need to look for a way to make it dynamic. So, unless your pal who's developing the API has finished their work, you're gonna need some serious help.

Here comes json-server, it allows you to mock the API, and provide dynamic access to data. By dynamic, I mean that we can read, add, update, and delete data (GET, POST, PUT, PATCH, DELETE).

It provides common URL use cases like:

and other miscellaneous stuff like:

Okay, let's get to it.

How to use it?

This will take less than 5 minutes!


  • NodeJS & npm
  • An API consumer (Your code, curl, postman, or simply your browser)


It's pretty straightforward to set up:

$ npm install -g json-server # Or with yarn $ yarn global add json-server # Then create a folder in which we will put our db.json file: $ mkdir blog && cd $_ # create our schema file $ touch db.json

To fill it in, we can either do it by hand or use a random json generator (my favorite is json-generator)

{ "articles": [ { "id": 1, "title": "Build an API using GO", "authorId": 2 }, { "id": 2, "title": "Build an API using API Platform", "authorId": 1 } ], "comments": [ { "id": 1, "body": "Brilliant", "articleId": 1 }, { "id": 2, "body": "Awesome", "articleId": 2 } ], "authors": [ { "id": 1, "username": "rpierlot", "title": "Romain Pierlot" }, { "id": 2, "username": "qneyrat", "title": "Quentin Neyrat" } ] }

Now we can run json-server so that we can access the endpoints that json-server created for us

$ json-server db.json \{^_^}/ hi! Loading db.json Done Resources http://localhost:3000/articles http://localhost:3000/comments http://localhost:3000/authors Home http://localhost:3000 Type s + enter at any time to create a snapshot of the database

Great, we've set up our API mock. Now we can test our endpoints:

$ curl http://localhost:3000/articles [ { "id": 1, "title": "Build an API using GO", "authorId": 2 }, { "id": 2, "title": "Build an API using API Platform", "authorId": 1 } ] $ curl http://localhost:3000/articles/1 { "id": 1, "title": "Build an API using GO", "authorId": 2 }


We can use almost all kinds of URIs to perform requests: For example, to insert (create) a new author, we might use: POST http://localhost:3000/authors

$ curl --data-urlencode "title=Vincent Composieux" --data "username=vcomposieux" http://localhost:3000/authors { "title": "Vincent Composieux", "username": "vcomposieux", "id": 3 }

To read an article with article id 2: GET http://localhost:3000/articles/2. The same URI would be used for PUT and DELETE, to update and delete, respectively.

Now, when it comes to creating a new comment in an article, one option might be: POST http://localhost:3000/comments And that could work to create a comment, but it's arguably outside the context of an article.

As a matter of fact, this URI is not as intuitive as it could be. It could be argued that the following URI would offer better clarity: POST http://localhost:3000/articles/1/comments. Now we know we're creating a comment in article id 1.

$ curl --data-urlencode "body=Cool article ;-)" http://localhost:3000/articles/1/comments { "body": "Cool article ;-)", "articleId": 1, "id": 4 }

Same with creating an article by author id 3:

$ curl --data-urlencode "title=GraphQL" http://localhost:3000/authors/3/articles { "title": "GraphQL", "authorId": "3", "id": 3 }

Filters, sort & operators

Filtering is done using query parameters: GET http://localhost:3000/articles?title=GraphQL.

Sort is as easy as adding _sort & _order (asc & desc) query parameters:

GET http://localhost:3000/articles?_sort=likes

(Assuming we have added the likes field to each article). The order is ascending by default.

In case we want to sort by multiple properties, we can write our properties separated by a comma:

GET http://localhost:3000/articles?_sort=author,score&_order=desc,asc

Operators are suffixes used to augment our query parameters:

  • _gt (greater than), _lt (less than), _gte (greater than or equal) and _lte (less than or equal): GET http://localhost:3000/comments?score_gte=5 (assuming we have a score field in the comments)
  • _ne(not equal) negation of an expression GET http://localhost:3000/comments?articleId_ne=2
  • _like is an operator that can be applied to string properties, it gives the same result as an SQL's LIKE. GET http://localhost:3000/articles?title_like=API


We can use the built-in query parameters _page & _limit to paginate our results. json-server exposes X-Total-Count and the Link header that contain links to the first, next and last pages.

GET http://localhost:3000/articles?_page=1&_limit=1

HTTP/1.1 200 OK X-Powered-By: Express Vary: Origin, Accept-Encoding Access-Control-Allow-Credentials: true Cache-Control: no-cache Pragma: no-cache Expires: -1 X-Total-Count: 3 Access-Control-Expose-Headers: X-Total-Count, Link Link: <http://localhost:3000/articles?_page=1&_limit=1>; rel="first", <http://localhost:3000/articles?_page=2&_limit=1>; rel="next", <http://localhost:3000/articles?_page=3&_limit=1>; rel="last" X-Content-Type-Options: nosniff Content-Type: application/json; charset=utf-8 Content-Length: 89 ETag: W/"59-24+hjZrVFdbtnn+FgcogU6QvujI" Date: Sun, 30 Jul 2017 17:22:34 GMT Connection: keep-alive

Full text search

We can implement a search feature in our app using full-text search by simply adding a q query parameter.

$ curl http://localhost:3000/articles?q=api [ { "id": 1, "title": "Build an API using GO", "author": "qneyrat" }, { "id": 2, "title": "Build an API using API Platform", "author": "rpierlot" } ]


We can see relationships using _embed & _expand parameters.

  • _embed allows us to see the children resources like comments: GET http://localhost:3000/articles?_embed=comments
  • _expand allows us to see the parent resources like articles: GET http://localhost:3000/comments?_expand=article
$ curl http://localhost:3000/articles?author=vincent&_embed=comments [ { "title": "GraphQL", "author": "vincent", "id": 3, "comments": [ { "body": "nice", "articleId": 3, "id": 3 }, { "body": "great!", "articleId": 3, "id": 4 } ] } ] $ curl http://localhost:3000/comments?_expand=article [ { "id": 1, "body": "Brilliant", "articleId": 1, "article": { "id": 1, "title": "Build an API using GO", "author": "qneyrat" } }, { "id": 2, "body": "Awesome", "articleId": 2, "article": { "id": 2, "title": "Build an API using API Platform", "author": "rpierlot" } }, ... ]

Until now, we've seen only the routes part of json-server, there are a lot more things we can do, let's see what's next.

Randomly generated data

The basic example in Typicode's docs presents a simple script that generates the users endpoint. Here we are going to write endpoints that serve randomly generated data using a data faker module. Personally, I use faker.js, but there are others that you can explore like Chance and Casual.

The random aspect of the generation occurs only once, and that's when we run the script. This means that json-server won't give us a different response for each request. Eventually, we have to install our fake data tool, then write the generation script:

$ yarn add faker $ touch generate.js

Keep in mind that the script must export a function that exclusively returns an object with keys (endpoints) inside.

// generate.js const faker = require('faker'); module.exports = () => ({ messages: [...Array(3)].map((value, index) => ({ id: index + 1, name: faker.hacker.noun(), status: faker.hacker.adjective(), description: faker.hacker.phrase(), })), });

Then run json-server by giving it the generation script:

$ json-server generate.js \{^_^}/ hi! Loading generate.js Done Resources http://localhost:3000/messages Home http://localhost:3000 Type s + enter at any time to create a snapshot of the database

And the results would be something like:

$ curl http://localhost:3000/messages [ { "id": 1, "name": "driver", "status": "cross-platform", "description": "If we connect the system, we can get to the ADP panel through the redundant PCI protocol!" }, { "id": 2, "name": "monitor", "status": "1080p", "description": "Try to synthesize the CSS driver, maybe it will navigate the bluetooth matrix!" }, { "id": 3, "name": "hard drive", "status": "virtual", "description": "Use the redundant SMS program, then you can compress the bluetooth port!" } ]

And we can still perform requests on it as we've seen in the routes section.

Custom routes

Let's imagine we are supposed to perform requests on several different endpoints on our future API, and these endpoints don't have the same URIs:

/api/dashboard /api/groups/ducks/stats /auth/users /rpierlot/articles

json-server allows us to specify route rewrites. We can address this problem by using a map that resolves the actual routes in our json schema:

{ "/api/:view": "/:view", "/api/groups/:planet/stats": "/stats?planet=:planet", "/:user/articles": "/articles?author=:user", "/auth/users": "/users" }

So, when we start json-server it shows us the route rewrites we are using :

$ json-server --watch db2.json --routes routes.json \{^_^}/ hi! Loading db2.json Loading routes.json Done Resources http://localhost:3000/users http://localhost:3000/dashboard http://localhost:3000/stats http://localhost:3000/articles Other routes /api/:view -> /:view /api/groups/:planet/stats -> /stats?planet=:planet /:user/articles -> /articles?author=:user /auth/users -> /users Home http://localhost:3000 Type s + enter at any time to create a snapshot of the database Watching...

Now we can perform our custom requests to see the results:

$ curl http://localhost:3000/api/dashboard { "visits": 3881, "views": 625128, "shares": 7862 } $ curl http://localhost:3000/api/groups/ducks/stats [ { "planet": "ducks", "stats": { "points": 5625, "ships": 8 } } ]


In case we want to augment our json-server instance with a specific behavior, we have the possibility to do so using custom middlewares, these middlewares are passed to the express server the same way we would do it when developing a classic express app. In this section, we're going to explore a useful example of a feature that is usually necessary.

Imagine we want to access a resource on the API, but it turns out that this resource is secured. We can say that it's just about data and assume that we'll be satisfied by just returning it, then we use json-server to provide the data without worrying about security. But, we know that something's odd, we want our app to be ready when our future API is ready, in order to test the whole thing together. So, instead of bypassing it, let's use middlewares to set up an authentication layer.

// auth.js const auth = require('basic-auth'); module.exports = (req, res, next) => { var user = auth(req); if (typeof user === 'undefined' || !== 'kamal' || user.pass !== 'secret') { // We will discuss this line later in this section. res.header('WWW-Authenticate', 'Basic realm="Access to the API"'); return res.status(401).send({ error: 'Unauthorized' }); } next(); };

Now run the json-server command with the --middlewares option:

$ json-server --watch db2.json --routes routes.json --middlewares auth.js

Notice: the --middlewares option accepts a list of files. --middlewares file1.js file2.js file3.js.

Now let's test our authentication layer:

$ curl http://localhost:3000/api/groups/ducks/stats { "error": "Unauthorized" }

And we can see json-server's log with the 401 HTTP status:

GET /api/groups/ducks/stats 401 12.180 ms - 29

When we display the headers of this response, we recognize this header WWW-Authenticate: Basic realm="Access to the API":

HTTP/1.1 401 Unauthorized X-Powered-By: Express Vary: Origin, Accept-Encoding Access-Control-Allow-Credentials: true Cache-Control: no-cache Pragma: no-cache Expires: -1 WWW-Authenticate: Basic realm="Access to the API" Content-Type: application/json; charset=utf-8 Content-Length: 29 ETag: W/"1d-t1Z3N2Fd2Yqi/vcyFQaHaMeQEew" Date: Thu, 03 Aug 2017 09:59:57 GMT Connection: keep-alive

Here is what Mozilla Developer Network tells about it:

The WWW-Authenticate and Proxy-Authenticate response headers define the authentication method that should be used to gain access to a resource. They need to specify which authentication scheme is used so that the client that wishes to authorize knows how to provide the credentials.

HTTP authentication : WWW-Authenticate and Proxy-Authenticate headers

Then we test again, and this time we add credentials to our request (Notice: curl's --user option is not restricted to basic authentication, we can do other types of authentication, see here):

$ curl --user kamal:secret http://localhost:3000/api/groups/ducks/stats [ { "planet": "ducks", "stats": { "points": 5625, "ships": 8 } } ]

Great! Obviously, it's a 200 HTTP status :-D

GET /api/groups/ducks/stats 200 4.609 ms - 94

As a NodeJS module

json-server is an express application, which means that we can use it in an existing node/express app to achieve special behaviors. Here is a simple example that shows how to customize the logger:

json-server uses morgan for logs, and the default format that it uses is the dev log format, which doesn't expose all the info that we want, we need to use the standard Apache combined log outpout format instead:

// server.js import express from 'express'; import api from './api'; const port = 9001; const app = express(); const API_ROOT = `http://localhost:${port}/api`; app.use('/api', api); app.listen(port, error => { if (error) { console.error(error); } else {'==> 🌎 Listening on port %s. Open up %s in your browser.', port, API_ROOT); } });
// api.js import { create, defaults, rewriter, router } from 'json-server'; import morgan from 'morgan'; import rewrites from './routes.json'; const server = create(); const apiEndpoints = router('db2.json'); // Deactivate the existing logger const middlewares = defaults({ logger: false }); // Here we use our own logging format server.use(morgan('combined', { colors: true })); server.use(rewriter(rewrites)); server.use(middlewares); server.use(apiEndpoints); export default server;

Then we run our server:

$ nodemon --exec babel-node server.js ==> 🌎 Listening on port 9001. Open up http://localhost:9001/api/ in your browser.

Here we can see our custom logs in the console:

$ curl --user kamal:secret http://localhost:9001/api/groups/ducks/stats ::1 - kamal [11/Aug/2017:15:04:58 +0000] "GET /api/groups/ducks/stats HTTP/1.1" 200 187 "-" "curl/7.51.0" # or with Chrome ::1 - - [10/Aug/2017:08:57:04 +0000] "GET /api/ HTTP/1.1" 200 - "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.115 Safari/537.36"


json-server has drastically decreased the time of scaffolding an API. Amongst the possibilities that we've seen, there are lots of use cases you can explore in order to use json-server, like logging customization, testing, reconciliation between micro-services, serverless applications ...etc.

I hope this post did shed some light on how we can use json-server. I tried to bring some useful use cases we encounter every day. If you still want to learn more about using it or even its inner working, I recommend exploring its github project.

Thanks for reading!

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