Jest test fails for async function that calls another async function

Question

I am trying to test an async function that uses data returned by another async function. Here is the code to explain my problem:

StudentInfo.js

export async function getData() {
  //studentData imported from another file
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(studentData);
    }, 5000);
  });
}

export async function filterStudents() {
  const studentData = await StudentInfo.getData();

  //computations

  return filteredData; //object
}

StudentInfo.test.js

import * as studentInfo from "src/StudentInfo.js"

describe("StudentInfo" , () => {
   test("student data correctly filtered", async () => {
      const studentData = [
         {
            name: Sarah Marshall,
            id: srhmar451
         },
         {...}
      ];
      expectedData = { [...], [...]};
      const spy = jest.spyOn(studentInfo, "getData");
      spy.mockReturnValue(studentData);
      await expect(studentInfo.filterStudents()).toEqual(expectedData);
   });
});

My test fails because the expected return value is Promise {}. Could someone help me write a test for my filterStudents() function? I have been stuck at this for too long.

Since you're spying on an async function, you should use spy.mockResolvedValue(studentData) — macphilips
– macphilips, Commented Jun 24, 2019 at 17:49

Brian Adams · Accepted Answer · 2019-06-27 05:15:18Z

Here is a working example to get you started:

StudentInfo.js

import * as StudentInfo from './StudentInfo';

export async function getData() {
  throw new Error('should not get here');  // <= getData will be mocked
}

export async function filterStudents() {
  const studentData = await StudentInfo.getData();
  return studentData.filter(v => v.id === '2');
}

StudentInfo.test.js

import * as StudentInfo from "./StudentInfo"

describe("StudentInfo", () => {
  test("student data correctly filtered", async () => {
    const studentData = [
      { name: 'student1', id: '1' },
      { name: 'student2', id: '2' },
      { name: 'student3', id: '3' }
    ];
    jest.spyOn(StudentInfo, "getData").mockResolvedValue(studentData);
    await expect(StudentInfo.filterStudents()).resolves.toEqual([{ name: 'student2', id: '2' }]);  // Success!
  });
});

You can use mockFn.mockResolvedValue to mock the return value as a Promise that resolves to the value you pass to it (although it also works fine if you just use mockFn.mockReturnValue since it is valid to call await on a value, for example const five = await 5; // <= works fine).

The important part is to use .resolves to tell Jest that the expected value is a Promise that you expect to resolve, and then chain the toEqual to assert on the expected resolved value.

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