@jrockwood/injected-console

Enables unit tests to easily mock the console.

Overview

There are plenty of console mocking libraries out there, but most of them take the same approach of enabling and disabling the mocking for the entire system. This can be really useful in some cases, but there are times where you may want to have finer-grained control over which parts of your application are mocked. Additionally, it can be useful to have explicit knowledge about which pieces of the application use the console. As such, the approach that this library takes is to create an interface which is then injected via a constructor or just a parameter into a function.

The abstract base class is ColoredConsole and two different classes provide the implementation: OsColoredConsole and VirtualColoredConsole.

Additionally, there are two closely-related classes that keep track of messages to log, but without colors. ErrorReporter simply keeps track of which messages were reported and ConsoleErrorReporter keeps track and also immediately sends it to the supplied console. ErrorReporter allows a function to report multiple errors/messages in a function, but without knowing or caring that these messages might be written to the console. The semantics of the message are stored (error vs. warning), but there is no implied UI for the message (print to the console vs. write to a file). Although there is no FileErrorReporter, one could be added very easily to support file logging. There are lots of logging libraries out there, so be careful in adding to this library that we're not trying to replicate a general-purpose logging library. It's meant to be simple.

Getting Started

Add a reference to your package.json file using npm or yarn.

yarn add @jrockwood/injected-console

Note that if you're using TypeScript, the type definitions are already included in this library and don't need to be added via npm/yarn.

Examples

Consuming an injected ColoredConsole

The following code shows an example of how to use the ColoredConsole interface inside of application code as an injected dependency.

code.ts

import { ColoredConsole, OsColoredConsole } from '@jrockwood/injected-console';

// By default callers don't need to specify a ColoredConsole to use and it will just default to the
// native `console` object. Unit tests will pass in a `VirtualColoredConsole` object to use.
export function printSomething(coloredConsole: ColoredConsole = new OsColoredConsole()) {
  coloredConsole.log('Some message', 'magenta');
}

Mocking the ColoredConsole in unit tests

Below is an example of how to construct a VirtualColoredConsole inside of a unit test so that nothing is actually printed to the console.

code.test.ts

import { VirtualColoredConsole } from '@jrockwood/injected-console';
import { printSomething } from './code';

describe('printSomething', () => {
  it('should have logged something', () => {
    const virtualConsole = new VirtualColoredConsole();
    printSomething(virtualConsole);
    expect(virtualConsole).toHaveJustLogged();
  });

  it('should write something specific', () => {
    const virtualConsole = new VirtualColoredConsole();
    printSomething(virtualConsole);
    expect(virtualConsole).toHaveJustLogged('Some message');
  });

  it('should verify all properties of all lines in oreder', () => {
    const virtualConsole = new VirtualColoredConsole();
    printSomething(virtualConsole);
    expect(virtualConsole.lines).toEqual([
      { type: 'log', message: 'Some message', foreColor: 'magenta', backColor: 'inherit' },
    ]);
  });
});

Using supplied Jasmine matchers

An additional @jrockwood/injected-console-matchers library provides custom Jasmine matchers to help in writing unit tests working with the VirtualColoredConsole. They're included by default when importing the library.

// -----------------------------------------------------------------------------
// Most common

/**
 * Verifies that the message matches, 'log' was used, and the fore and back
 * colors are 'inherit'.
 */
expect(coloredConsole).toHaveJustLogged(string? | RegExp?, ignoreColors?: boolean);

/**
 * Verifies that the message matches, 'success' was used, the fore color is 'green', and
 * the back color is 'inherit'.
 */
expect(coloredConsole).toHaveJustLoggedSuccess(string? | RegExp?, ignoreColors?: boolean);

/**
 * Verifies that the last message matches, 'error' was used, the fore color is
 * 'red', and the back color is 'inherit'.
 */
expect(coloredConsole).toHaveJustLoggedError(string? | RegExp?, ignoreColors?: boolean);

/**
 * Verifies that the last message matches, 'warn' was used, the fore color is
 * 'yellow', and the back color is 'inherit'.
 */
expect(coloredConsole).toHaveJustLoggedWarning(string? | RegExp?, ignoreColors?: boolean);

/**
 * Verifies that the last message matches, 'info' was used, the fore color is
 * 'inherit', and the back color is 'inherit'.
 */
expect(coloredConsole).toHaveJustLoggedInfo(string? | RegExp?, ignoreColors?: boolean);

// -----------------------------------------------------------------------------
// Checking existence of messages (unordered)

expect(coloredConsole).toHaveLine(string? | RegExp?);