@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

Installing

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

Requirements

• TypeScript^3.0.0

Usage

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;
}

//Create:
var 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

var 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

Verifying calls

calculator.enabled = true;
var 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));

Ignoring all arguments

//ignoring all arguments
calculator.add(Arg.all()).returns(10);
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(Arg.all()).returns(10);
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);
calculator.add(Arg.all()).returns(10);
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;
}

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

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

//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);
fakeCalculator.divide(Arg.all()).returns(1338);
console.log(fakeCalculator.divide(10, 5)); //prints 5
console.log(fakeCalculator.divide(9, 5)); //prints 1338

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.

Beware

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) {
    console.log(someNumber);
  }
}

var 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.
Substitute.disableFor(fake).received(1337);

//now we can assert that we received a call to the "received" method.
fake.received().received(1337);

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
    }
}