tessl/npm-jest-fetch-mock

Comprehensive fetch API mocking solution for Jest testing environments with configurable responses and TypeScript support

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Overview

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Files

Response Mocking

Name: tessl/npm-jest-fetch-mock
Author: tessl

Core functionality for mocking successful HTTP responses with configurable bodies and options. Supports both one-time and persistent mocks with flexible response generation.

Capabilities

Mock Response

Sets up a mock response for all subsequent fetch calls until reset or changed.

/**
 * Mock response for all subsequent fetch calls
 * @param bodyOrFunction - Response body string or function that generates response
 * @param init - Optional response configuration (status, headers, etc.)
 * @returns FetchMock instance for chaining
 */
fetch.mockResponse(bodyOrFunction, init);

Usage Examples:

// Simple string response
fetch.mockResponse("Hello World");

// JSON response with status
fetch.mockResponse(JSON.stringify({ data: "test" }), { 
  status: 200,
  headers: { "Content-Type": "application/json" }
});

// Dynamic response function
fetch.mockResponse((request) => {
  if (request.url.includes("/api/users")) {
    return Promise.resolve(JSON.stringify({ users: [] }));
  }
  return Promise.resolve("Not found");
});

Mock Response Once

Sets up a mock response for the next fetch call only. Subsequent calls will use the default behavior or other configured mocks.

/**
 * Mock response for the next fetch call only
 * @param bodyOrFunction - Response body string or function that generates response
 * @param init - Optional response configuration (status, headers, etc.)
 * @returns FetchMock instance for chaining
 */
fetch.mockResponseOnce(bodyOrFunction, init);

Usage Examples:

// Mock single response
fetch.mockResponseOnce(JSON.stringify({ id: 1, name: "Alice" }));

// Chain multiple one-time responses
fetch
  .mockResponseOnce(JSON.stringify({ page: 1 }))
  .mockResponseOnce(JSON.stringify({ page: 2 }));

// With custom status and headers
fetch.mockResponseOnce("Server Error", { 
  status: 500,
  statusText: "Internal Server Error",
  headers: { "Content-Type": "text/plain" }
});

Once (Alias)

Convenient alias for mockResponseOnce providing the same functionality with shorter syntax.

/**
 * Alias for mockResponseOnce - mock response for the next fetch call only
 * @param bodyOrFunction - Response body string or function that generates response
 * @param init - Optional response configuration (status, headers, etc.)
 * @returns FetchMock instance for chaining
 */
fetch.once(bodyOrFunction, init);

Usage Examples:

// Same as mockResponseOnce
fetch.once(JSON.stringify({ success: true }));

// Chaining with alias
fetch
  .once("First response")
  .once("Second response")
  .once("Third response");

Mock Responses

Sets up multiple mock responses that will be used in sequence. Each response is used once, in the order provided.

/**
 * Mock multiple responses in sequence
 * @param responses - Array of responses (strings, arrays with [body, init], or functions)
 * @returns FetchMock instance for chaining
 */
fetch.mockResponses(...responses);

Usage Examples:

// Multiple string responses
fetch.mockResponses(
  "First response",
  "Second response", 
  "Third response"
);

// Mixed response types with configurations
fetch.mockResponses(
  [JSON.stringify({ page: 1 }), { status: 200 }],
  [JSON.stringify({ page: 2 }), { status: 200 }],
  ["Not found", { status: 404 }]
);

// Using functions
fetch.mockResponses(
  (req) => Promise.resolve(JSON.stringify({ url: req.url })),
  "Static response",
  [JSON.stringify({ final: true }), { status: 200 }]
);

Types

/**
 * Function type for generating dynamic responses
 */
type MockResponseInitFunction = (request: Request) => Promise<MockResponseInit | string>;

/**
 * Response configuration interface
 */
interface MockResponseInit extends MockParams {
  body?: string;
  init?: MockParams;
}

/**
 * Basic response parameters
 */
interface MockParams {
  status?: number;
  statusText?: string;
  headers?: string[][] | { [key: string]: string };
  url?: string;
}