Tutorial: Count assertions

While Mocha itself does not provide an assertion layer and cannot provide assertion counting, it's relatively easy to integrate this behavior using hooks. The following is a simplified version of an assertion counter:

var expected = 0;
var actual = 0;

function assert(expr, msg) {
  if (!expr) throw new Error(msg || "assertion failed");
  actual++;
}

function expect(n) {
  expected = n;
}

function reset() {
  expected = 0;
  actual = 0;
}

function check() {
  if (!expected || expected == actual) return;
  var err = new Error("expected " + expected + " assertions, got " + actual);
  this.currentTest.emit("error", err);
}

beforeEach(reset);
afterEach(check);

describe("something", function () {
  it("should work", function (done) {
    expect(2);

    setTimeout(function () {
      assert("wahoo");
    }, 50);

    setTimeout(function () {
      assert("hey");
    }, 50);

    setTimeout(function () {
      done();
    }, 100);
  });
});