API Mocking

Simulating API behavior to enable development and testing without real backend services.

API mocking is the practice of creating simulated versions of APIs that mimic the behavior of real services. It enables teams to develop and test applications independently of actual backend systems.

Why Mock APIs?

Faster Development: No waiting for backend teams to finish
Reliable Testing: Consistent responses without network issues
Cost Reduction: Avoid third-party API charges during development
Edge Case Testing: Easily simulate errors and unusual scenarios

Mocking Approaches

Approach	Description	Best For
Static Mocks	Fixed JSON responses	Simple testing
Dynamic Mocks	Generated based on request	Realistic scenarios
Record & Replay	Capture real responses	Integration testing
Contract-Based	From OpenAPI/Swagger specs	API-first development

Popular Tools

MSW - Mock Service Worker for browser/Node
Nock - HTTP mocking for Node.js
Mirage JS - Client-side API mocking
Prism - OpenAPI-based mock server

Code Examples

MSW Handler

import { http, HttpResponse } from "msw";

export const handlers = [
  http.get("/api/users", () => {
    return HttpResponse.json([
      { id: 1, name: "John" },
      { id: 2, name: "Jane" }
    ]);
  })
];

Related Terms

OpenAPI Specification

A standard format for describing REST APIs, enabling documentation and code generation.

WebSocket

A protocol enabling full-duplex, real-time communication between client and server.

Latency

The time delay between a request being sent and a response being received.

Server-Sent Events (SSE)

A standard for pushing real-time updates from server to client over HTTP.