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How to spy on an inner function from same file with Jest

I have the following functions file:
// functions.js
function bar() {
return 'bar';
}
function foo() {
return bar();
}
export functions = {
bar,
foo,
};
And I want to test the foo function as following:
// functions.test.js
import {functions} from './functions.js';
describe('test foo', () => {
it('should call bar', () => {
const spy = jest.spyOn(functions, 'bar');
functions.foo();
expect(spy).toHaveBeenCalled();
});
});
And I get the following error:
Expected number of calls: >= 1
Received number of calls: 0
expect(spy).toHaveBeenCalled();
^
Any idea what is wrong with this test?

You are spying on the wrong function.
functions.bar is not the same bar that is called inside the foo function.
foo is referencing bar that is a standalone function, however you are spying on a function that is attached to the functions object.
If you change your code like this it will work.
export functions = {
bar(){},
foo(){this.bar()},
};

How should I mock a specific function in a module? [duplicate]

What's the best way to correctly mock the following example?
The problem is that after import time, foo keeps the reference to the original unmocked bar.
module.js:
export function bar () {
return 'bar';
}
export function foo () {
return `I am foo. bar is ${bar()}`;
}
module.test.js:
import * as module from '../src/module';
describe('module', () => {
let barSpy;
beforeEach(() => {
barSpy = jest.spyOn(
module,
'bar'
).mockImplementation(jest.fn());
});
afterEach(() => {
barSpy.mockRestore();
});
it('foo', () => {
console.log(jest.isMockFunction(module.bar)); // outputs true
module.bar.mockReturnValue('fake bar');
console.log(module.bar()); // outputs 'fake bar';
expect(module.foo()).toEqual('I am foo. bar is fake bar');
/**
* does not work! we get the following:
*
* Expected value to equal:
* "I am foo. bar is fake bar"
* Received:
* "I am foo. bar is bar"
*/
});
});
I could change:
export function foo () {
return `I am foo. bar is ${bar()}`;
}
to:
export function foo () {
return `I am foo. bar is ${exports.bar()}`;
}
but this is pretty ugly in my opinion to do everywhere.

An alternative solution can be importing the module into its own code file and using the imported instance of all of the exported entities. Like this:
import * as thisModule from './module';
export function bar () {
return 'bar';
}
export function foo () {
return `I am foo. bar is ${thisModule.bar()}`;
}
Now mocking bar is really easy, because foo is also using the exported instance of bar:
import * as module from '../src/module';
describe('module', () => {
it('foo', () => {
spyOn(module, 'bar').and.returnValue('fake bar');
expect(module.foo()).toEqual('I am foo. bar is fake bar');
});
});
Importing the module into its own code looks strange, but due to the ES6's support for cyclic imports, it works really smoothly.

The problem seems to be related to how you expect the scope of bar to be resolved.
On one hand, in module.js you export two functions (instead of an object holding these two functions). Because of the way modules are exported the reference to the container of the exported things is exports like you mentioned it.
On the other hand, you handle your export (that you aliased module) like an object holding these functions and trying to replace one of its function (the function bar).
If you look closely at your foo implementation you are actually holding a fixed reference to the bar function.
When you think you replaced the bar function with a new one you just actually replaced the reference copy in the scope of your module.test.js
To make foo actually use another version of bar you have two possibilities :
In module.js export a class or an instance, holding both the foo and bar method:
Module.js:
export class MyModule {
function bar () {
return 'bar';
}
function foo () {
return `I am foo. bar is ${this.bar()}`;
}
}
Note the use of this keyword in the foo method.
Module.test.js:
import { MyModule } from '../src/module'
describe('MyModule', () => {
//System under test :
const sut:MyModule = new MyModule();
let barSpy;
beforeEach(() => {
barSpy = jest.spyOn(
sut,
'bar'
).mockImplementation(jest.fn());
});
afterEach(() => {
barSpy.mockRestore();
});
it('foo', () => {
sut.bar.mockReturnValue('fake bar');
expect(sut.foo()).toEqual('I am foo. bar is fake bar');
});
});
Like you said, rewrite the global reference in the global exports container. This is not a recommended way to go as you will possibly introduce weird behaviors in other tests if you don't properly reset the exports to its initial state.

fwiw, the solution I settled on was to use dependency injection, by setting a default argument.
So I would change
export function bar () {
return 'bar';
}
export function foo () {
return `I am foo. bar is ${bar()}`;
}
to
export function bar () {
return 'bar';
}
export function foo (_bar = bar) {
return `I am foo. bar is ${_bar()}`;
}
This is not a breaking change to the API of my component, and I can easily override bar in my test by doing the following
import { foo, bar } from '../src/module';
describe('module', () => {
it('foo', () => {
const dummyBar = jest.fn().mockReturnValue('fake bar');
expect(foo(dummyBar)).toEqual('I am foo. bar is fake bar');
});
});
This has the benefit of leading to slightly nicer test code too :)

I had this same problem and due to the project's linting standards, defining a class or rewriting references in the exports were not code review approvable options even if not prevented by the linting definitions. What I stumbled on as a viable option is to use the babel-rewire-plugin which is much cleaner, at least in appearance. While I found this used in another project I had access to, I noticed it was already in an answer in a similar question which I have linked here. This is a snippet adjusted for this question (and without using spies) provided from the linked answer for reference (I also added semicolons in addition to removing spies because I'm not a heathen):
import __RewireAPI__, * as module from '../module';
describe('foo', () => {
it('calls bar', () => {
const barMock = jest.fn();
__RewireAPI__.__Rewire__('bar', barMock);
module.foo();
expect(bar).toHaveBeenCalledTimes(1);
});
});
https://stackoverflow.com/a/45645229/6867420

Works for me:
cat moduleWithFunc.ts
export function funcA() {
return export.funcB();
}
export function funcB() {
return false;
}
cat moduleWithFunc.test.ts
import * as module from './moduleWithFunc';
describe('testFunc', () => {
beforeEach(() => {
jest.clearAllMocks();
});
afterEach(() => {
module.funcB.mockRestore();
});
it.only('testCase', () => {
// arrange
jest.spyOn(module, 'funcB').mockImplementationOnce(jest.fn().mockReturnValue(true));
// act
const result = module.funcA();
// assert
expect(result).toEqual(true);
expect(module.funcB).toHaveBeenCalledTimes(1);
});
});

From this thread:
Try using a function expression
export const bar = () => {
return "bar"
}
This should let you spy on bar even if its used by another function in the same module.

If you define your exports you can then reference your functions as part of the exports object. Then you can overwrite the functions in your mocks individually. This is due to how the import works as a reference, not a copy.
module.js:
exports.bar () => {
return 'bar';
}
exports.foo () => {
return `I am foo. bar is ${exports.bar()}`;
}
module.test.js:
describe('MyModule', () => {
it('foo', () => {
let module = require('./module')
module.bar = jest.fn(()=>{return 'fake bar'})
expect(module.foo()).toEqual('I am foo. bar is fake bar');
});
})

If you're using Babel (i.e. #babel/parser) to handle transpiling your code, the babel-plugin-explicit-exports-references1 npm package solves this pretty elegantly by making the "ugly" module.exports replacements for you transparently at transpile time. See the original problem thread for more information.
1 Note: I wrote this plugin!

For CommonJS modules users, suppose the file looks something like:
/* myModule.js */
function bar() {
return "bar";
}
function foo() {
return `I am foo. bar is ${bar()}`;
}
module.exports = { bar, foo };
You need to modify the file to:
/* myModule.js */
function bar() {
return "bar";
}
function foo() {
return `I am foo. bar is ${myModule.bar()}`; // Change `bar()` to `myModule.bar()`
}
const myModule = { bar, foo }; // Items you wish to export
module.exports = myModule; // Export the object
Your original test suite (myModule.test.js) should now pass:
const myModule = require("./myModule");
describe("myModule", () => {
test("foo", () => {
jest.spyOn(myModule, "bar").mockReturnValueOnce("bar-mock");
const result = myModule.foo();
expect(result).toBe("I am foo. bar is bar-mock");
});
});
Read more: Mock/Spy exported functions within a single module in Jest

There are various hacks available here to make this work, but the real answer most people should be using is: don't. Taking the OP's example module:
export function bar () {
return 'bar';
}
export function foo () {
return `I am foo. bar is ${bar()}`;
}
and testing the actual behaviour, you'd write:
import { bar, foo } from "path/to/module";
describe("module", () => {
it("foo returns 'bar'", () => {
expect(bar()).toBe('bar');
});
it("foo returns 'I am foo. bar is bar'", () => {
expect(foo()).toBe('I am foo. bar is bar');
});
});
Why? Because then you can refactor inside the module boundary without changing the tests, which gives you the confidence to improve the quality of your code in the knowledge that it still does what it's supposed to.
Imagine you extracted the creation of 'bar' from bar to an unexported function, for example:
function rawBar() {
return 'bar';
}
export function bar () {
return rawBar();
}
export function foo () {
return `I am foo. bar is ${rawBar()}`;
}
The test I suggest above would pass. If you'd asserted that calling foo meant bar got called, that test would start failing, even though the refactor preserved the module's behaviour (same API, same outputs). That's an implementation detail.
Test doubles are for collaborators, if something really does need to be mocked here it should be extracted to a separate module (then mocking it is much easier, which tells you you're moving in the right direction). Trying to mock functions in the same module is like mocking parts of a class you're trying to test, which I illustrate similarly here: https://stackoverflow.com/a/66752334/3001761.

VS code intellisense with es6 modules

Is there a way to make VS Code intellisense work when a module method is instantiated this way :
lib.js :
export function foo() {
this.aMethod = function() {
console.log('bar');
}
}
main.js :
import { foo } from "./lib.js";
function bar() {
this.newFoo = new foo();
this.newFoo. <-- no intellisense here, "aMethod" is not suggested...
}
Am I doing something wrong ?

The proper way of defining your class is either by doing
export function foo() {
this.aMethod = function() {
console.log('bar');
}
}
or
export function foo() {
}
foo.prototype.aMethod = function() {
console.log('bar');
}
VSCode should be able to detect your object's method after that

What is the syntax to export a function from a module in Node.js?

What is the syntax to export a function from a module in Node.js?
function foo() {}
function bar() {}
export foo; // I don't think this is valid?
export default bar;

In Node you export things with module.exports special object. For example:
This exports both functions:
module.exports = { foo, bar };
They can be used as:
const { foo, bar } = require('./module/path');
To export one of those functions as top-level object you can use:
module.exports = foo;
module.exports.bar = bar;
which can be used as:
const foo = require('./module/path');
and:
const { bar } = require('./module/path');
or:
const foo = require('./module/path');
const { bar } = foo;
or:
const foo = require('./module/path');
const bar = foo.bar;
etc.
This is "the syntax to export a function from a module in Node.js" as asked in the question - i.e. the syntax that is natively supported by Node. Node doesn't support import/export syntax (see this to know why). As slezica pointed put in the comments below you can use a transpiler like Babel to convert the import/export keywords to syntax understood by Node.
See those answers for more info:
Is it ok to use Babel npm package for node.js server application
javascript - Why is there a spec for sync and async modules?

to expose both foo and bar functions:
module.exports = {
foo: function() {},
bar: function() {}
}

You can also do this in a shorter form
// people.js
function Foo() {
// ...
}
function Bar() {
// ...
}
module.exports = { Foo, Bar}
Importing:
// index.js
const { Foo, Bar } = require('./people.js');

export function foo(){...};
Or, if the function has been declared earlier:
export {foo};
Reference: MDN export

How to mock functions in the same module using Jest?

What's the best way to correctly mock the following example?
The problem is that after import time, foo keeps the reference to the original unmocked bar.
module.js:
export function bar () {
return 'bar';
}
export function foo () {
return `I am foo. bar is ${bar()}`;
}
module.test.js:
import * as module from '../src/module';
describe('module', () => {
let barSpy;
beforeEach(() => {
barSpy = jest.spyOn(
module,
'bar'
).mockImplementation(jest.fn());
});
afterEach(() => {
barSpy.mockRestore();
});
it('foo', () => {
console.log(jest.isMockFunction(module.bar)); // outputs true
module.bar.mockReturnValue('fake bar');
console.log(module.bar()); // outputs 'fake bar';
expect(module.foo()).toEqual('I am foo. bar is fake bar');
/**
* does not work! we get the following:
*
* Expected value to equal:
* "I am foo. bar is fake bar"
* Received:
* "I am foo. bar is bar"
*/
});
});
I could change:
export function foo () {
return `I am foo. bar is ${bar()}`;
}
to:
export function foo () {
return `I am foo. bar is ${exports.bar()}`;
}
but this is pretty ugly in my opinion to do everywhere.

An alternative solution can be importing the module into its own code file and using the imported instance of all of the exported entities. Like this:
import * as thisModule from './module';
export function bar () {
return 'bar';
}
export function foo () {
return `I am foo. bar is ${thisModule.bar()}`;
}
Now mocking bar is really easy, because foo is also using the exported instance of bar:
import * as module from '../src/module';
describe('module', () => {
it('foo', () => {
spyOn(module, 'bar').and.returnValue('fake bar');
expect(module.foo()).toEqual('I am foo. bar is fake bar');
});
});
Importing the module into its own code looks strange, but due to the ES6's support for cyclic imports, it works really smoothly.

The problem seems to be related to how you expect the scope of bar to be resolved.
On one hand, in module.js you export two functions (instead of an object holding these two functions). Because of the way modules are exported the reference to the container of the exported things is exports like you mentioned it.
On the other hand, you handle your export (that you aliased module) like an object holding these functions and trying to replace one of its function (the function bar).
If you look closely at your foo implementation you are actually holding a fixed reference to the bar function.
When you think you replaced the bar function with a new one you just actually replaced the reference copy in the scope of your module.test.js
To make foo actually use another version of bar you have two possibilities :
In module.js export a class or an instance, holding both the foo and bar method:
Module.js:
export class MyModule {
function bar () {
return 'bar';
}
function foo () {
return `I am foo. bar is ${this.bar()}`;
}
}
Note the use of this keyword in the foo method.
Module.test.js:
import { MyModule } from '../src/module'
describe('MyModule', () => {
//System under test :
const sut:MyModule = new MyModule();
let barSpy;
beforeEach(() => {
barSpy = jest.spyOn(
sut,
'bar'
).mockImplementation(jest.fn());
});
afterEach(() => {
barSpy.mockRestore();
});
it('foo', () => {
sut.bar.mockReturnValue('fake bar');
expect(sut.foo()).toEqual('I am foo. bar is fake bar');
});
});
Like you said, rewrite the global reference in the global exports container. This is not a recommended way to go as you will possibly introduce weird behaviors in other tests if you don't properly reset the exports to its initial state.

fwiw, the solution I settled on was to use dependency injection, by setting a default argument.
So I would change
export function bar () {
return 'bar';
}
export function foo () {
return `I am foo. bar is ${bar()}`;
}
to
export function bar () {
return 'bar';
}
export function foo (_bar = bar) {
return `I am foo. bar is ${_bar()}`;
}
This is not a breaking change to the API of my component, and I can easily override bar in my test by doing the following
import { foo, bar } from '../src/module';
describe('module', () => {
it('foo', () => {
const dummyBar = jest.fn().mockReturnValue('fake bar');
expect(foo(dummyBar)).toEqual('I am foo. bar is fake bar');
});
});
This has the benefit of leading to slightly nicer test code too :)

I had this same problem and due to the project's linting standards, defining a class or rewriting references in the exports were not code review approvable options even if not prevented by the linting definitions. What I stumbled on as a viable option is to use the babel-rewire-plugin which is much cleaner, at least in appearance. While I found this used in another project I had access to, I noticed it was already in an answer in a similar question which I have linked here. This is a snippet adjusted for this question (and without using spies) provided from the linked answer for reference (I also added semicolons in addition to removing spies because I'm not a heathen):
import __RewireAPI__, * as module from '../module';
describe('foo', () => {
it('calls bar', () => {
const barMock = jest.fn();
__RewireAPI__.__Rewire__('bar', barMock);
module.foo();
expect(bar).toHaveBeenCalledTimes(1);
});
});
https://stackoverflow.com/a/45645229/6867420

Works for me:
cat moduleWithFunc.ts
export function funcA() {
return export.funcB();
}
export function funcB() {
return false;
}
cat moduleWithFunc.test.ts
import * as module from './moduleWithFunc';
describe('testFunc', () => {
beforeEach(() => {
jest.clearAllMocks();
});
afterEach(() => {
module.funcB.mockRestore();
});
it.only('testCase', () => {
// arrange
jest.spyOn(module, 'funcB').mockImplementationOnce(jest.fn().mockReturnValue(true));
// act
const result = module.funcA();
// assert
expect(result).toEqual(true);
expect(module.funcB).toHaveBeenCalledTimes(1);
});
});

From this thread:
Try using a function expression
export const bar = () => {
return "bar"
}
This should let you spy on bar even if its used by another function in the same module.

If you define your exports you can then reference your functions as part of the exports object. Then you can overwrite the functions in your mocks individually. This is due to how the import works as a reference, not a copy.
module.js:
exports.bar () => {
return 'bar';
}
exports.foo () => {
return `I am foo. bar is ${exports.bar()}`;
}
module.test.js:
describe('MyModule', () => {
it('foo', () => {
let module = require('./module')
module.bar = jest.fn(()=>{return 'fake bar'})
expect(module.foo()).toEqual('I am foo. bar is fake bar');
});
})

If you're using Babel (i.e. #babel/parser) to handle transpiling your code, the babel-plugin-explicit-exports-references1 npm package solves this pretty elegantly by making the "ugly" module.exports replacements for you transparently at transpile time. See the original problem thread for more information.
1 Note: I wrote this plugin!

For CommonJS modules users, suppose the file looks something like:
/* myModule.js */
function bar() {
return "bar";
}
function foo() {
return `I am foo. bar is ${bar()}`;
}
module.exports = { bar, foo };
You need to modify the file to:
/* myModule.js */
function bar() {
return "bar";
}
function foo() {
return `I am foo. bar is ${myModule.bar()}`; // Change `bar()` to `myModule.bar()`
}
const myModule = { bar, foo }; // Items you wish to export
module.exports = myModule; // Export the object
Your original test suite (myModule.test.js) should now pass:
const myModule = require("./myModule");
describe("myModule", () => {
test("foo", () => {
jest.spyOn(myModule, "bar").mockReturnValueOnce("bar-mock");
const result = myModule.foo();
expect(result).toBe("I am foo. bar is bar-mock");
});
});
Read more: Mock/Spy exported functions within a single module in Jest

There are various hacks available here to make this work, but the real answer most people should be using is: don't. Taking the OP's example module:
export function bar () {
return 'bar';
}
export function foo () {
return `I am foo. bar is ${bar()}`;
}
and testing the actual behaviour, you'd write:
import { bar, foo } from "path/to/module";
describe("module", () => {
it("foo returns 'bar'", () => {
expect(bar()).toBe('bar');
});
it("foo returns 'I am foo. bar is bar'", () => {
expect(foo()).toBe('I am foo. bar is bar');
});
});
Why? Because then you can refactor inside the module boundary without changing the tests, which gives you the confidence to improve the quality of your code in the knowledge that it still does what it's supposed to.
Imagine you extracted the creation of 'bar' from bar to an unexported function, for example:
function rawBar() {
return 'bar';
}
export function bar () {
return rawBar();
}
export function foo () {
return `I am foo. bar is ${rawBar()}`;
}
The test I suggest above would pass. If you'd asserted that calling foo meant bar got called, that test would start failing, even though the refactor preserved the module's behaviour (same API, same outputs). That's an implementation detail.
Test doubles are for collaborators, if something really does need to be mocked here it should be extracted to a separate module (then mocking it is much easier, which tells you you're moving in the right direction). Trying to mock functions in the same module is like mocking parts of a class you're trying to test, which I illustrate similarly here: https://stackoverflow.com/a/66752334/3001761.