Jest - mock function in 'child_process' package - unit-testing

Question

I'm writing unit tests and which to mock the 'exec' method in package 'child_process'.
__mocks__/child_process.js
const child_process = jest.genMockFromModule('child_process');
child_process.exec = jest.fn()
module.exports = child_process;
This is the test file:
const fs = require('fs-extra'),
child_process = require('child_process'),
runCassandraMigration = require('../../lib/runCassandraMigration.js')
const defaultArguments = () => {
return {
migration_script_path: './home',
logger: {
error: function () {}
}
};
}
jest.mock("fs-extra")
jest.mock("child_process")
describe('Running cassandra migration tests', function () {
describe('successful flow', function () {
it('Should pass without any errors ', async function () {
let args = defaultArguments();
let loggerSpy = jest.spyOn(args.logger, 'error')
fs.remove.mockImplementation(() => {Promise.resolve()})
child_process.exec.mockImplementation(() => {Promise.resolve()})
await runCassandraMigration(args.migration_script_path, args.logger)
});
});
When I run the test I get the following error:
child_process.exec.mockImplementation is not a function
The module I test
const fs = require('fs-extra')
const promisify = require('util').promisify
const execAsync = promisify(require('child_process').exec)
module.exports = async (migration_script_path, logger) => {
try {
console.log()
const {stdout, stderr} = await execAsync(`cassandra-migration ${migration_script_path}`)
logger.info({stdout: stdout, stderr: stderr}, 'Finished runing cassandra migration')
await fs.remove(migration_script_path)
} catch (e) {
logger.error(e, 'Failed to run cassandra migration')
throw Error()
}
}
Please advise.

Answer 1

A late... answer?...
Yesterday I got the same error and the problem was that I wasn't calling jest.mock('child_process') in my test file.
Jest documentation says that when mocking Node's core modules calling jest.mock('child_process') is required. I see you do this but for some reason it is not working (maybe Jest is not hoisting it to the top).
Anyways, with Jest version 24.9.0 I don't get the child_process.exec.mockImplementation is not a function error but get some other errors because your test is not well implemented.
To make your test work I:
Added info: function () {}, inside logger
Updated the implementation of exec to child_process.exec.mockImplementation((command, callback) => callback(null, {stdout: 'ok'}))
And also (not necessary for the test to pass) updated the implementation of fs.remove to fs.remove.mockImplementation(() => Promise.resolve())
Like this:
const fs = require('fs-extra'),
child_process = require('child_process'),
runCassandraMigration = require('./stack')
const defaultArguments = () => {
return {
migration_script_path: './home',
logger: {
info: function () {},
error: function () {}
}
};
}
jest.mock("fs-extra")
jest.mock("child_process")
describe('Running cassandra migration tests', function () {
describe('successful flow', function () {
it('Should pass without any errors ', async function () {
let args = defaultArguments();
let loggerSpy = jest.spyOn(args.logger, 'error')
fs.remove.mockImplementation(() => Promise.resolve())
child_process.exec.mockImplementation((command, callback) => callback(null, {stdout: 'ok'}))
await runCassandraMigration(args.migration_script_path, args.logger)
});
});
});