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1.208.0 • Public • Published

What is this?

@fluffy-spoon/substitute is a TypeScript port of NSubstitute, which aims to provide a much more fluent mocking opportunity for strong-typed languages.

You can read an in-depth comparison of substitute.js versus other popular TypeScript mocking frameworks here: https://medium.com/@mathiaslykkegaardlorenzen/with-typescript-3-and-substitute-js-you-are-already-missing-out-when-mocking-or-faking-a3b3240c4607

PRs are very welcome! Help is much appreciated.


npm install @fluffy-spoon/substitute --save-dev


  • TypeScript^3.0.0


import { Substitute, Arg } from '@fluffy-spoon/substitute';

interface Calculator {
  add(a: number, b: number): number;
  subtract(a: number, b: number): number;
  divide(a: number, b: number): number;
  async heavyOperation(): Promise<number>;

  isEnabled: boolean;

// Create:
const calculator = Substitute.for<Calculator>();
// Set a return value:
calculator.add(1, 2).returns(3);
// Check received calls:
calculator.received().add(1, Arg.any());
calculator.didNotReceive().add(2, 2);

Creating a mock

const calculator = Substitute.for<Calculator>();

Setting return types

See the example below. The same syntax also applies to properties and fields.

// single return type
calculator.add(1, 2).returns(4);
console.log(calculator.add(1, 2)); // prints 4
console.log(calculator.add(1, 2)); // prints undefined

// multiple return types in sequence
calculator.add(1, 2).returns(3, 7, 9);
console.log(calculator.add(1, 2)); // prints 3
console.log(calculator.add(1, 2)); // prints 7
console.log(calculator.add(1, 2)); // prints 9
console.log(calculator.add(1, 2)); // prints undefined

Working with promises

When working with promises you can also use resolves() and rejects() to return a promise.

calculator.heavyOperation(1, 2).resolves(4); 
// same as calculator.heavyOperation(1, 2).returns(Promise.resolve(4));
console.log(await calculator.heavyOperation(1, 2)); // prints 4
calculator.heavyOperation(1, 2).rejects(new Error());
// same as calculator.heavyOperation(1, 2).returns(Promise.reject(new Error()));
console.log(await calculator.heavyOperation(1, 2)); // throws Error

Verifying calls

calculator.enabled = true;
const foo = calculator.add(1, 2);

// verify call to add(1, 2)
calculator.received().add(1, 2);

// verify property set to "true"
calculator.received().enabled = true;

Argument matchers

There are several ways of matching arguments. The examples below also applies to properties and fields - both when setting up calls and verifying them.

Matching specific arguments

import { Arg } from '@fluffy-spoon/substitute';

// ignoring first argument
calculator.add(Arg.any(), 2).returns(10);
console.log(calculator.add(1337, 3)); // prints undefined since second argument doesn't match
console.log(calculator.add(1337, 2)); // prints 10 since second argument matches

// received call with first arg 1 and second arg less than 0
calculator.received().add(1, Arg.is(x => x < 0));

Generic and inverse matchers

import { Arg } from '@fluffy-spoon/substitute';

const equalToZero = (x: number) => x === 0;

// first argument will match any number
// second argument will match a number that is not '0'
calculator.divide(Arg.any('number'), Arg.is.not(equalToZero)).returns(10);
console.log(calculator.divide(100, 10)); // prints 10

const argIsNotZero = Arg.is.not(equalToZero);
calculator.received(1).divide(argIsNotZero, argIsNotZero);

Note: Arg.is() will automatically infer the type of the argument it's replacing

Ignoring all arguments

// ignoring all arguments
console.log(calculator.add(1, 3)); // prints 10
console.log(calculator.add(5, 2)); // prints 10

Match order

The order of argument matchers matters. The first matcher that matches will always be used. Below are two examples.

calculator.add(1, 3).returns(1337);
console.log(calculator.add(1, 3)); // prints 10
console.log(calculator.add(5, 2)); // prints 10
calculator.add(1, 3).returns(1337);
console.log(calculator.add(1, 3)); // prints 1337
console.log(calculator.add(5, 2)); // prints 10

Partial mocks

With partial mocks you always start with a true substitute where everything is mocked and then opt-out of substitutions in certain scenarios.

import { Substitute, Arg } from '@fluffy-spoon/substitute';

class RealCalculator implements Calculator {
  add(a: number, b: number) => a + b;
  subtract(a: number, b: number) => a - b;
  divide(a: number, b: number) => a / b;

const realCalculator = new RealCalculator();
const fakeCalculator = Substitute.for<Calculator>();

// let the subtract method always use the real method
console.log(fakeCalculator.subtract(20, 10)); // prints 10
console.log(fakeCalculator.subtract(1, 2)); // prints -1

// for the add method, we only use the real method when the first arg is less than 10
// else, we always return 1337
fakeCalculator.add(Arg.is(x < 10), Arg.any()).mimicks(realCalculator.add);
fakeCalculator.add(Arg.is(x >= 10), Arg.any()).returns(1337);
console.log(fakeCalculator.add(5, 100)); // prints 105 via real method
console.log(fakeCalculator.add(210, 7)); // prints 1337 via fake method

// for the divide method, we only use the real method for explicit arguments
fakeCalculator.divide(10, 2).mimicks(realCalculator.divide);
console.log(fakeCalculator.divide(10, 5)); // prints 5
console.log(fakeCalculator.divide(9, 5)); // prints 1338

Throwing exceptions

Exceptions can be thrown on properties or methods. You can add different exceptions for different arguments

import { Substitute, Arg } from '@fluffy-spoon/substitute';

interface Calculator {
  add(a: number, b: number): number;
  subtract(a: number, b: number): number;
  divide(a: number, b: number): number;
  isEnabled: boolean;

const calculator = Substitute.for<Calculator>();
calculator.divide(Arg.any(), 0).throws(new Error('Cannot divide by 0'));
calculator.divide(1, 0); // throws the exception Error: Cannot divide by 0

Benefits over other mocking libraries

  • Easier-to-understand fluent syntax.
  • No need to cast to any in certain places (for instance, when overriding read-only properties) due to the myProperty.returns(...) syntax.
  • Doesn't weigh much.
  • Produces very clean and descriptive error messages. Try it out - you'll love it.
  • Doesn't rely on object instances - you can produce a strong-typed fake from nothing, ensuring that everything is mocked.


Names that conflict with Substitute.js

Let's say we have a class with a method called received, didNotReceive or mimick keyword - how do we mock it?

Simple! We disable the proxy methods temporarily while invoking the method by using the disableFor method which disables these special methods.

class Example {
  received(someNumber: number) {

const fake = Substitute.for<Example>();

// BAD: this would have called substitute.js' "received" method.
// fake.received(2);

// GOOD: we now call the "received" method we have defined in the class above.

// now we can assert that we received a call to the "received" method.

Strict mode

If you have strict set to true in your tsconfig.json, you may need to toggle off strict null checks. The framework does not currently support this.

However, it is only needed for your test projects anyway.

    "compilerOptions": {
        "strict": true,
        "strictNullChecks": false


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