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Version: 1.9.0

Using express middlewares


This tutorial assumes that you have completed the "Definitions using javascript" tutorial.

You have now static fixtures defined using javascript, but, what if you want your /api/users/:id api url to respond with the correspondent user without the need of changing the current behavior?

This is usually not recommended, because you are going to implement almost a "real api", and maybe it should be better to shutdown the Mocks Server and connect the application to your real api, but for some special cases maybe you need to accomplish it.

Let's see how it can be done using express middlewares.

Define the initial users collection

Extract the users collection response from your javascript "get-users" fixture, because it is going to be reused also by the new fixture:


id: 1,
name: "John Doe"
id: 2,
name: "Jane Doe"

const getUsers = {
id: "get-users",
url: "/api/users",
method: "GET",
response: {
status: 200,


Add an express middleware fixture

Add a fixture for GET /api/users/:id that will respond with the user with correspondent id, or a "not found" error if any user matches:



const getUserReal = {
id: "get-user-real",
url: "/api/users/:id",
method: "GET",
response: (req, res) => {
const userId =;
const user = INITIAL_USERS.find(userData => === Number(userId));
if (user) {
} else {
message: "User not found"

module.exports = {

Fixtures "response" functions are called with express "request", "response" and "next" methods. Read the express documentation to learn more about req, res, next.

Add a new behavior

Add a new behavior extending the "standard" one, and adding the getUserReal fixture:

// /mocks/behaviors.js

const { Behavior } = require("@mocks-server/main");

const { getUsers, getUser2 } = require("./fixtures/users");

const standard = new Behavior([
], {
id: "standard"

const user2 = standard.extend([ getUser2 ], {
id: "user2"

const dynamic = standard.extend([ getUserReal ], {
id: "dynamic"

module.exports = [ standard, user2, dynamic ];

Change current behavior

Now you'll have three behaviors available: "standard", "user2" and "dynamic". Use the CLI to select the "dynamic" one.

Available behaviors

Check the responses

Browse to http://localhost:3100/api/users/1. You should see the first user:

{"id":1,"name":"John Doe"}

Browse to http://localhost:3100/api/users/2. You should now see the second user:

{"id":2,"name":"Jane Doe"}

Browse to http://localhost:3100/api/users/3:

{"message":"User not found"}


You could add also express middleware fixtures for deleting, updating, or creating users, simply modifying the INITIAL_USERS memory object from each correspondent response function.

Changes will be be persisted in memory while the server is running.