Promitto

Simple Promise mocking for unit testing.

const p = promitto.pending("Some API results");

render(<MyComponent data={p} />);

// assert loading state

await p.resolve().settled();

// assert final state

Getting Started

npm i --save-dev @jasonewall/promitto

import promitto from '@jasonewall/promitto'

API

promitto<T>()

Returns a promise mock that can be resolved or rejected by calling either:

1. resolve(value?: T) - The promise will resolve to the value passed in, and value will be passed to any fulfillment handlers attached to the promise mock.
2. reject(reason?: any) - The promise will reject with the reason passed in and reason will be passed to any rejection handlers attached to the promise mock.

No then, catch, finally callbacks will be executed until either of these are called.

const p = promitto<string>();

p.resolve("Testing is fun!");

p.reject(new Error("Error handling is fun!"));

promitto.pending<T>(value: T)

Returns a promise mock that will not run any callbacks (added through then, catch, finally) until resolve() is called on the promise mock. The wrapped value of the promise mock is locked into the value initially passed into pending.

const p = promitto.pending("Hello!");

p.resolve();

promitto.resolve(value?: T)

Returns a promise mock that is already resolved to the provided value. Any callbacks added by then, catch, or finally will be immediately executed.

const p = promitto.resolve(["Cats", "Dogs"]);

promitto.reject(reason?: any)

Return a promise mock that is already rejected to the provided value. Any callbacks added by then, catch, or finally will be imediately executed.

const p = promitto.reject(new Error("Missing pets!"));

See Use Cases for more info.

PromiseMock Instance Methods

#children: PromiseMock<any>

Returns all promises created by calling then, catch, finally of this PromiseMock.

const p = promitto.pending("Some value");

callMyApplication(p);

console.log(p.children);

[More Info]

#settled(): Promise<T>

Returns a promise that only settles once this promise and all of it's children are settled.

const p = promitto.resolve("Good job!");

callMyApplication(p);

await p.settled();

// assert application state

Settling Rejected PromiseMocks

[More Info]

#status: PromiseState

Returns the current status of this PromiseMock. Will be one of:

enum PromiseState {
  Pending = "pending",
  Fulfilled = "fulfilled",
  Rejected = "rejected",
}

Compatability with Core JS/TS Promise

This is a huge concern of Promitto. We DO NOT want our tests to be buggy because our mocks are creating promise chains that behave differently than real promises. To ensure Promitto PromiseMocks behave exactly like real promises we have a large test suite that compares the behaviour of PromiseMock to Promise.

In short - we run our test suite against Promise core to test the tests. See the Promise tests for more details.

If you feel there are some cases that are missing Pull Requests and/or Issues describing the missing test cases are welcome.

For Testing Only

As a mostly synchronous promise implemenation this library has no value for application code. Please only use it for testing.