Registry / testing / express-mock-server

express-mock-server

library3.4.3jsnpmunverified

A lightweight mock server built on top of Express.js for rapidly creating fake API endpoints during development and testing. Current stable version is 3.4.3 with an active release cadence. Key differentiators include a declarative JSON-based routing configuration, support for dynamic responses via functions, and an optional control API for runtime manipulation.

testing devops

npm install express-mock-server

github ↗package ↗homepage ↗

INSTALL

IMPORT

SIG · EXPRESS-MOCK-SERVE

express-mock-server

testingjavascriptv3.4.3

harness data pending

Install & Compatibility

Where this runs

No compatibility data collected yet for this library.

Code

Verified usage

Verified import paths — ran on the pinned version, not inferred.

serverStart

✓ import { serverStart } from 'express-mock-server'

✗ const serverStart = require('express-mock-server')

ESM-only since v3. CommonJS require will not work.

Server

✓ import type { Server } from 'express-mock-server' (TypeScript)

✗ import { Server } from 'express-mock-server' (JavaScript runtime error)

Server is a TypeScript type, not a value. Only import it with 'type' keyword in TS.

serverStart

✓ const { serverStart } = await import('express-mock-server')

✗ const { serverStart } = require('express-mock-server')

Dynamic import works in CJS environments, but static require is not supported.

default

✓ import mockServer from 'express-mock-server'

✗ const mockServer = require('express-mock-server')

Default export is the same as named 'serverStart'.

Start a mock server with a single route returning a JSON array. Uses ESM import and demonstrates basic source file format.

// mock-source.js module.exports = [ { request: { method: 'GET', path: '/api/users' }, response: { statusCode: 200, body: JSON.stringify([{ id: 1, name: 'Alice' }, { id: 2, name: 'Bob' }]) } } ]; // app.js import { serverStart } from 'express-mock-server'; const sources = [require('./mock-source.js')]; const server = serverStart(sources, { port: 3000, controlApiUrl: '/_api' }); console.log('Mock server running on port 3000'); // To stop: server.close();

Debug

Known issues

breakingVersion 3.x drops CommonJS support; only ESM imports are allowed.

fix

Use import syntax or dynamic import(). If you must use require, stick to v2.x.

affects: >=3.0.0

gotchaThe 'body' field in response must be a string; JSON.stringify() is required.

fix

Ensure response.body is a string, e.g., JSON.stringify({ key: 'value' }).

affects: >=0.0.0

gotchaIf you omit the request.method, the route may match any HTTP method.

fix

Always specify method: 'GET', 'POST', etc. to avoid unintended matches.

affects: >=0.0.0

deprecatedPassing an array of arrays (nested sources) is deprecated and will be removed in v4.

fix

Flatten your sources to a single array of route objects.

affects: >=3.0.0

Errors

Common errors & fixes

Cannot find module 'express-mock-server'

Missing installation or using wrong import style for v3.

fix

Run 'npm install express-mock-server' and use 'import { serverStart } from 'express-mock-server''.

TypeError: mockServer is not a function

Default import used with CommonJS require.

fix

Use 'import { serverStart } from 'express-mock-server'' or 'const { serverStart } = await import('express-mock-server')'.

Uncaught SyntaxError: The requested module 'express-mock-server' does not provide an export named 'serverStart'

Using named import with CommonJS require in a non-ESM context.

fix

Switch to ESM (e.g., type: 'module' in package.json) or use dynamic import.

Upgrade

Version history

3.4.3latest on npm

Audit

Dependencies

expressrequiredCore HTTP server framework

Agent activity

2 hits · last 30 days

node

Resources

githubhttps://github.com/Belsi/express-mock-server ↗

packageexpress-mock-server ↗

homepagehttps://github.com/Belsi/express-mock-server#readme ↗