express-api-mockr

Powerful Express middleware to mock REST APIs with dynamic responses, per-route latency simulation, and request-aware body generation. Perfect for frontend development without a real backend.

Features

• Route-Based Mocking -- Define responses by URL path and HTTP method (GET, POST, PUT, DELETE).
• Dynamic Responses -- Use functions to generate responses based on the incoming request object.
• Latency Simulation -- Add global or per-route delays to simulate real network latency.
• Status Codes -- Return any HTTP status code per route.
• Verbose Logging -- Optional console logging of matched routes for debugging.
• Express Native -- Drop-in middleware compatible with Express 4 and 5.

Installation

npm install express-api-mockr

Usage

Basic Setup

const express = require('express');
const apiMock = require('express-api-mockr');

const app = express();
app.use(express.json());

app.use(apiMock({
  endpoints: {
    '/api/users': {
      get: {
        status: 200,
        body: [
          { id: 1, name: 'Alice', role: 'Admin' },
          { id: 2, name: 'Bob', role: 'User' },
          { id: 3, name: 'Charlie', role: 'Editor' }
        ]
      }
    }
  }
}));

app.listen(3000, () => {
  console.log('Mock server running on http://localhost:3000');
});

Dynamic Responses

Use a function for the body property to generate responses based on the request.

app.use(apiMock({
  endpoints: {
    '/api/users': {
      post: {
        status: 201,
        body: (req) => ({
          id: Math.floor(Math.random() * 10000),
          name: req.body.name,
          email: req.body.email,
          createdAt: new Date().toISOString()
        })
      }
    },
    '/api/search': {
      get: {
        body: (req) => ({
          query: req.query.q,
          results: [],
          timestamp: Date.now()
        })
      }
    }
  }
}));

Simulating Latency

app.use(apiMock({
  delay: 500, // Global 500ms delay for all routes
  endpoints: {
    '/api/fast': {
      get: {
        delay: 0, // Override: no delay for this route
        body: { message: 'instant response' }
      }
    },
    '/api/slow': {
      get: {
        delay: 3000, // Override: 3 second delay for this route
        body: { message: 'slow response' }
      }
    }
  }
}));

Error Simulation

app.use(apiMock({
  endpoints: {
    '/api/protected': {
      get: {
        status: 401,
        body: { error: 'Unauthorized', message: 'Token required' }
      }
    },
    '/api/broken': {
      default: {
        status: 500,
        body: { error: 'Internal Server Error' }
      }
    }
  }
}));

Verbose Logging

app.use(apiMock({
  verbose: true, // Logs "[Mocker] Matching GET /api/users" to console
  endpoints: { /* ... */ }
}));

Fallthrough Behavior

If a request does not match any defined endpoint, the middleware calls next() and passes control to the next middleware or route handler in the stack.

API Reference

apiMock(config)

| Option | Type | Default | Description | | --- | --- | --- | --- | | endpoints | Object | {} | Route definitions keyed by path | | delay | number | 0 | Global delay in milliseconds | | verbose | boolean | false | Log matched routes to console |

Endpoint Definition

{
  '/api/path': {
    get: { status: 200, body: { ... }, delay: 0 },
    post: { status: 201, body: (req) => ({ ... }) },
    default: { status: 404, body: { error: 'Not found' } }
  }
}

License

MIT -- see LICENSE for details.