Jest:Mock

lastmod 2026-11-10created 2022-04-113 edits

Jest:Mock is a page summarizing the mocking features of Jest, a JavaScript testing framework. jest.mock() replaces an entire module with a fake implementation, jest.fn() creates a mock function that records its calls, and mockResolvedValue or mockRejectedValue specifies asynchronous results. It is used to make tests deterministic by removing external API calls and network dependencies.

Example

import axios from "axios";

import { BASE_URL, fetchUsers } from "./utils";

jest.mock("axios");

describe("fetchUsers", () => {
  describe("when API call is successful", () => {
    it("should return users list", async () => {
      // given
      const users = [
        { id: 1, name: "John" },
        { id: 2, name: "Andrew" },
      ];
      axios.get.mockResolvedValueOnce(users);

      // when
      const result = await fetchUsers();

      // then
      expect(axios.get).toHaveBeenCalledWith(`${BASE_URL}/users`);
      expect(result).toEqual(users);
    });
  });

  describe("when API call fails", () => {
    it("should return empty users list", async () => {
      // given
      const message = "Network Error";
      axios.get.mockRejectedValueOnce(new Error(message));

      // when
      const result = await fetchUsers();

      // then
      expect(axios.get).toHaveBeenCalledWith(`${BASE_URL}/users`);
      expect(result).toEqual([]);
    });
  });
});

Jest:Mock

Example

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