Interceptor

A library that helps with testing asynchronous function calls.

Installation

npm install --save @tjenkinson/interceptor

or available on JSDelivr at "https://cdn.jsdelivr.net/npm/@tjenkinson/interceptor@1".

API

intercept(fn)

This takes a function to intercept and returns an object with the following properties:

fn

A function which takes the same arguments as the input fn, and returns a promise. If the input fn returned a promise then this has the same signature and can be used in its place.

nextCall(): Promise

When the returned fn is called it will be proxied to the input fn and the returned promise will resolve.

nextCall(callback: ({ input, makeCall, sendResult }) => T): Promise

This takes a callback which will be called synchronously when the input fn is called. If fn has already been called then it will be called immediately for the oldest call.

It receives an object with the following properties:

• callArgs: An array containing the arguments passed to the fn call.
• makeCall: A function which calls the input fn with the original arguments and passes through the return value.
• sendResult: A function which takes a value that the promise returned from the input fn call should resolve with.

The promise returned from this function resolves with the return value from the callback.

numCallsWaiting(): number

A function which returns the number of calls that have not been handled with nextCall yet.

Example

This example uses jest. Other frameworks are availble ;)

Waiting for a function call

import { intercept } from '@tjenkinson/interceptor';

function doSomethingAsync(callback) {
  setTimeout(() => callback('it works'), 200);
}

it('calls the callback with the correct value', async () => {
  const fn = jest.fn();
  const intercepted = intercept(fn);

  doSomethingAsync(intercepted.fn);

  await intercepted.nextCall();
  expect(fn).toHaveBeenCalledWith('it works');
});