koa-mock-response

Mocks give you full control over the response headers and body returned to the client. They can be used to return anything from a simple html string to a resourceful REST API. Typically, they're used to mock services but can be used for anything.

In the config, define an array called mocks. Each mock definition maps a route to a response. A simple home page:

{
  "mocks": [
    {
      "route": "/",
      "response": {
        "body": "<h1>Welcome to the Mock Responses example</h1>"
      }
    }
  ]
}

Under the hood, the property values from the response object are written onto the underlying koa response object. You can set any valid koa response properies, for example type:

{
  "mocks": [
    {
      "route": "/",
      "response": {
        "type": "text/plain",
        "body": "<h1>Welcome to the Mock Responses example</h1>"
      }
    }
  ]
}

Conditional Response

To define a conditional response, set a request object on the mock definition. The request value acts as a query - the response defined will only be returned if each property of the request query matches. For example, return an XML response only if the request headers include accept: application/xml, else return 404 Not Found.

{
  "mocks": [
    {
      "route": "/two",
      "request": { "accepts": "xml" },
      "response": {
        "body": "<result id='2' name='whatever' />"
      }
    }
  ]
}

Multiple Potential Responses

To specify multiple potential responses, set an array of mock definitions to the responses property. The first response with a matching request query will be sent. In this example, the client will get one of two responses depending on the request method:

{
  "mocks": [
    {
      "route": "/three",
      "responses": [
        {
          "request": { "method": "GET" },
          "response": {
            "body": "<h1>Mock response for 'GET' request on /three</h1>"
          }
        },
        {
          "request": { "method": "POST" },
          "response": {
            "status": 400,
            "body": { "message": "That method is not allowed." }
          }
        }
      ]
    }
  ]
}

Dynamic Response

The examples above all returned static data. To define a dynamic response, create a mock module. Specify its path in the module property:

{
  "mocks": [
    {
      "route": "/four",
      "module": "/mocks/stream-self.js"
    }
  ]
}

Here's what the stream-self module looks like. The module should export a mock definition (an object, or array of objects, each with a response and optional request). In this example, the module simply streams itself to the response but you could set body to any valid value.

const fs = require('fs')
 
module.exports = {
  response: {
    body: fs.createReadStream(__filename)
  }
}

Response function

For more power, define the response as a function. It will receive the koa context as its first argument. Now you have full programmatic control over the response returned.

module.exports = {
  response: function (ctx) {
    ctx.body = '<h1>I can do anything i want.</h1>'
  }
}

If the route contains tokens, their values are passed to the response. For example, with this mock...

{
  "mocks": [
    {
      "route": "/players/:id",
      "module": "/mocks/players.js"
    }
  ]
}

...the id value is passed to the response function. For example, a path of /players/10?name=Lionel would pass 10 to the response function. Additional, the value Lionel would be available on ctx.query.name:

module.exports = {
  response: function (ctx, id) {
    ctx.body = `<h1>id: ${id}, name: ${ctx.query.name}</h1>`
  }
}

RESTful Resource example

Here's an example of a REST collection (users). We'll create two routes, one for actions on the resource collection, one for individual resource actions.

{
  "mocks": [
    { "route": "/users", "module": "/mocks/users.js" },
    { "route": "/users/:id", "module": "/mocks/user.js" }
  ]
}

Define a module (users.json) defining seed data:

[
  { "id": 1, "name": "Lloyd", "age": 40, "nationality": "English" },
  { "id": 2, "name": "Mona", "age": 34, "nationality": "Palestinian" },
  { "id": 3, "name": "Francesco", "age": 24, "nationality": "Italian" }
]

The collection module:

const users = require('./users.json')
 
/* responses for /users */
const mockResponses = [
  /* Respond with 400 Bad Request for PUT and DELETE - inappropriate on a collection */
  { request: { method: 'PUT' }, response: { status: 400 } },
  { request: { method: 'DELETE' }, response: { status: 400 } },
  {
    /* for GET requests return a subset of data, optionally filtered on 'minAge' and 'nationality' */
    request: { method: 'GET' },
    response: function (ctx) {
      ctx.body = users.filter(user => {
        const meetsMinAge = (user.age || 1000) >= (Number(ctx.query.minAge) || 0)
        const requiredNationality = user.nationality === (ctx.query.nationality || user.nationality)
        return meetsMinAge && requiredNationality
      })
    }
  },
  {
    /* for POST requests, create a new user and return the path to the new resource */
    request: { method: 'POST' },
    response: function (ctx) {
      const newUser = ctx.request.body
      users.push(newUser)
      newUser.id = users.length
      ctx.status = 201
      ctx.response.set('Location', `/users/${newUser.id}`)
    }
  }
]
 
module.exports = mockResponses

The individual resource module:

const users = require('./users.json')
 
/* responses for /users/:id */
const mockResponses = [
  /* don't support POST here */
  { request: { method: 'POST' }, response: { status: 400 } },
 
  /* for GET requests, return a particular user */
  {
    request: { method: 'GET' },
    response: function (ctx, id) {
      ctx.body = users.find(user => user.id === Number(id))
    }
  },
 
  /* for PUT requests, update the record */
  {
    request: { method: 'PUT' },
    response: function (ctx, id) {
      const updatedUser = ctx.request.body
      const existingUserIndex = users.findIndex(user => user.id === Number(id))
      users.splice(existingUserIndex, 1, updatedUser)
      ctx.status = 200
    }
  },
 
  /* DELETE request: remove the record */
  {
    request: { method: 'DELETE' },
    response: function (ctx, id) {
      const existingUserIndex = users.findIndex(user => user.id === Number(id))
      users.splice(existingUserIndex, 1)
      ctx.status = 200
    }
  }
]
 
module.exports = mockResponses