Detecting console.log() calls

If you're using Jasmine, it's dead simple:

it('is my test', function () {
    spyOn(console, 'log');
    // do your stuff that should log something
    expect(console.log).toHaveBeenCalledWith('something');
});

Head to Jasmine docs for more info.

console.log doesn't keep a record of messages that are logged, or emit any events that you could listen for. It's not possible for your tests to directly verify its output from JavaScript. Instead, your test code will need to replace console.log with a mock implementation that does keep track of log messages for later verification.

Mocking is a common feature supported by most JavaScript test frameworks. For example, the Jest test framework provides a jest.spyOn function which replaces a given method with a mock implementation that records the arguments for each call in a .mock property before passing them on to the original implementation. After each test you may want to call jest.clearAllMocks() to reset the recorded argument lists for the next test, or use the equivalent clearMocks: true config option.

function saySomething() {
  console.log("Hello World");
}

jest.spyOn(console, 'log');

test("saySomething says hello", () => {
  expect(console.log.mock.calls.length).toBe(0);
  saySomething();
  expect(console.log.mock.calls.length).toBe(1);
  expect(console.log.mock.calls[0][0]).toBe("Hello World");
});

afterEach(() => {
  jest.clearAllMocks();
});

If you're not using a test framework (you probably should), you can create a simple mock yourself.

function saySomething() {
  console.log("Hello World");
}

function testSomething() {
  // Replace console.log with stub implementation.
  const originalLog = console.log;
  const calls = [];
  console.log = (...args) => {
    calls.push(args);
    originalLog(...args);
  };

  try {
    console.assert(calls.length == 0);
    saySomething();
    console.assert(calls.length == 1);
    console.assert(calls[0][0] == "Hello World");
  } catch (error) {
    console.error(error);
  } finally {
    // Restore original implementation after testing.
    console.log = originalLog;
  }
}

So not bad solutions, but if you're looking for a high powered logger try Paul Irish's log()

If that's too high powered, you can get by with something like this.

var console = window.console,
    _log = console ? console.log : function(){};

_log.history = [];

console.log = function( ){
  _log.history.push.apply( _log.history, arguments );
  _log.apply( console, arguments );
}

Usage

console.log('I','have','an','important','message');
//Use native one instead
_log.call( console, _log.history );

http://jsfiddle.net/BeXdM/