Test Simple Logger Functions With Full Code Coverage

I'm using Chai, Sinon and Instanbul to test a NodeJS application. Here's the Logger code: import Debug, { IDebugger } from 'debug'; export default class Logger { private readonl

Solution 1:

log isn't covered because it never got called, you stubbed it out with sinon.stub(Logger.prototype, 'log').

Your current implementation is difficult to test because Logger is too coupled to Debug. Creating instances of dependencies in the constructor is generally not a good idea.

If you invert the dependency, making Logger's constructor take an IDebugger instead of the string name:

importDebug, { IDebugger } from'debug';

exportdefaultclassLogger {

  constructor(privatereadonly dbg: IDebugger) {}

  publiclog(str: string): void {
    this.dbg(str);
  }
}

then when you test Logger, you can easily inject a test double:

it("debugs the input", () => {
  const debug = sinon.stub();
  const logger = newLogger(debug);
  const message = "hello, world!";

  logger.log(message);

  sinon.assert.calledWithExactly(debug, message);
});

This means you don't need to spy on any part of Logger itself, allowing you to refactor inside that class more easily. The Logger class now only depends on the abstract IDebugger interface, not the concrete Debug implementation.

Creating a Logger instance with a Debug given the specific name can just be a function, or a static method:

exportdefaultclassLogger {

  staticwithName(name: string) {
    returnnewLogger(Debug(name));
  }

  /* ... */
}