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1.3.0 • Public • Published

Typescript Mock Imports

Intuitive mocking for Typescript imports.

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ts-mock-imports leverages the ES6 import syntax to mock out imported code with stub versions of the imported objects. This allows ES6 code to be easily unit-tested without the need for an explicit dependency injection library.

ts-mock-imports is built on top of sinon. Sinon stub documentation

Mocked classes take all of the original class functions, and replace them with noop functions (functions returning undefined) while maintaining type safety.

This library needs to be run on TypeScript 2.6.1 or later.


ts-mock-imports is built on top of Sinon and TypeScript. Ensure you have both installed.

npm install typescript
npm install sinon --save-dev

Install the library

npm install ts-mock-imports --save-dev



export class Foo {
  private count: number;
  constructor() {
    throw new Error();
  public getCount(): number {
    return count;


import { Foo } from './foo';
export class Bar {
  constructor() {
    const foo = new Foo();


import { ImportMock } from 'ts-mock-imports';
import { Bar } from './Bar';
import * as fooModule from '../src/foo';
// Throws error
const bar = new Bar();
const mockManager = ImportMock.mockClass(fooModule, 'Foo');
// No longer throws an error
const bar = new Bar();
// Easily add mock responses for testing
mockmanager.mock('getCount', 3)
// Call restore to reset all mocked objects to original imports



mockClass(module: <import * as>, importName?: string ): MockManager<T>


The module containing the class you would like to mock.

Both the source file and test file need to use the same path to import the mocked module. I.e. Cannot use 'src/index' to import into the .spec.ts file and then use 'src/foo' to import into bar.ts. Both files need to use either 'src/foo' or 'src/index'.


What the class is exported as. If exported using export default then this parameter is not needed.

Using importName:

// export class Foo
import * as fooModule from '../src/foo';
const mockManager = ImportMock.mockClass(fooModule, 'Foo');

Default imports:

// export default Foo
import * as foo from '../foo';
const mockManager = ImportMock.mockClass(foo);

Import mock will infer the type of Foo if it is the only item exported out of it's file. If more things are exported, you will need to explicitly provide types to Import mock.

Explicit typing:

import * as fooModule from '../foo';
const mockManager = ImportMock.mockClass<fooModule.Foo>(fooModule, 'Foo');

If you wish to ensure that Foo is the correct name for the mocked class, give import mock the type of your module.

Explicit typing with full type assurance

import * as fooModule from '../foo';
const mockManager = ImportMock.mockClass<fooModule.Foo, typeof fooModule>(fooModule, 'Foo');
// Will result in a TS Error as Bar is not exported by Foo
const mockManager = ImportMock.mockClass<fooModule.Foo, typeof fooModule>(fooModule, 'Bar');

mockClass replaces the original export with a fake class. All original functions exist on the fake class as noop functions (functions returning undefined).

mockStaticClass(module: <import * as>, importName?: string ): MockStaticManager<T>

Takes the same arguments as mockClass but only replaces static functions on the original class.

Static classes: (Only recreates static methods)

import * as fooModule from '../foo';
const mockManager = ImportMock.mockStaticClass(fooModule, 'Foo');

mockFunction(module: <import * as>, importName?: string, returns?: any): SinonStub

Returns a SinonStub that is set up to return the optional argument.

Call restore on the stub object to restore the original export.

Function exports:

import * as fooModule from '../foo';
const stub = ImportMock.mockFunction(fooModule, 'fooFunction', 'bar');
// fooFunction will now return bar

mockOther(module: <import * as>, importName?: string, replaceWith: Partial<typeof module.importName>): OtherManager<T>

mockOther() uses the replaceWith argument to entirely replace the original exported item.

Useful for mocking out or removing variables and enums.

Variable mocking:

import * as fooModule from '../foo';
const mockManager = ImportMock.mockOther(fooModule, 'fooName', 'fakeName');
// import { fooName } from './foo' now returns 'fakeName'


Requires an object that matches Partial. This argument is an optional shorthand, and the value can be updated using mockMangaer.set().

restore(): void

restore() will restore all mocked items. Allows ImportMock to be used as a sandbox.

Useful for restoring when multiple mocks have been created.

Variable mocking:

import * as fooModule from '../foo';
import * as bazModule from '../baz';
ImportMock.mockClass(fooModule, 'Foo');
ImportMock.mockClass(fooModule, 'Bar');
ImportMock.mockFunction(bazModule, 'mainFunction')
// <run tests>
// all mocked imports will now be restored to their original values

MockManager (and MockStaticManager)

MockManager<T>.mock(functionName: string, returns?: any): SinonStub

Returns a sinon stub object.


The name of the function you would like to mock.

If using MockManager, Typescript expects the functionName to match functions available on the original class.

MockStaticManager allows any string.


The value returned when the mocked function is called.

Mocking functions: (Returns a sinon stub)

import * as fooModule from '../foo';
const fooManager = ImportMock.mockClass(fooModule, 'Foo');
// Will throw a type error if bar() does not exist on Foo
// new Foo().bar() will return undefined

Mocking functions with a return object:

import * as fooModule from '../foo';
const mockManager = ImportMock.mockClass(fooModule, 'Foo');
mockManager.mock('bar', 'Bar');
// new Foo().bar() now returns 'Bar'

If you wish to run modified code when the mocked function is called, you can use sinon.callsFake()

const mockManager = ImportMock.mockClass(fooModule, 'Foo');
const sinonStub = mockManager.mock('bar');
sinonStub.callsFake(() => {
  // custom code here

MockManager<T>.set(varName: string, replaceWith?: any): void

Replaces a property with a given value.


The name of the property you would like to mock.

If using MockManager, Typescript expects the varName to match properties available on the original class.

MockStaticManager allows any string.


The mock value of the property.

Mocking variable with a return object:

import * as fooModule from '../foo';
const mockManager = ImportMock.mockClass(fooModule, 'Foo');
const newVal = 5;
mockManager.set('count', newVal);
// new Foo().count now returns 5

MockManager<T>.getMockInstance(): T

Returns an instance of the mocked class.

import * as fooModule from '../foo';
const mockManager = ImportMock.mockClass(fooModule, 'Foo');
const sinonStub = mockManager.mock('bar', 'Bar');
const mockFoo = mockManager.getMockInstance(); // returns 'Bar'


Restores the import back to the original class.

It is important that this is called so future imports work as expected.


OtherManager<T>.set(replaceWith?: T): void

Replaces an exported property with a given value.

This value must match the type of the original export.


The mock value of the export.

Mocking variable with a return object:

import * as fooModule from '../foo';
const mockManager = ImportMock.mockOther(fooModule, 'FooName', 'fakeName');
// import { FooName } from './foo' imports 'fakeName'
const newVal = 'newName';
// import { FooName } from './foo' now imports 'newName'

OtherManager<T>.getValue(): T

Returns the current mockValue

import * as fooModule from '../foo';
const mockManager = ImportMock.mockOther(fooModule, 'FooName', 'fakeName');
mockManager.getValue(); // returns 'fakeName'


Restores the import back to the original class.

It is important that this is called so future imports work as expected.


Import mock works best when mocking es6 exports. Due to JavaScript's sometimes winding development history, there are some modules that use alternate export patterns that may not work correctly when mocked using Import mock. To reduce the chance of issues, all production code should aim to use import { item } from 'module'; syntax. This allows the test code to use import * as object from 'module'; syntax seamlessly.

Requirejs is not currently compatible with this library.


This library contains two types of tests.

  1. Typescript tests to ensure typing works as intended: npm run dtslint
  2. Unit tests to check the runtime functionality of the library: npm run unit-test

Both test suites are run when using npm run test


npm i ts-mock-imports

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