Assertion planning

[Turbo87]

It might be useful to have assertion planning built into chai for async testing (see e.g. avajs/ava#25).

I currently imagine it working roughly like this:

describe('something', function() {
    afterEach(function() {
        chai.verifyPlan();
    });

    it('works', function() {
        chai.plan(2);

        return somethingAsync().then(() => {
            expect(true).to.be.true;
            expect([1, 2, 3])
                .to.contain(1)
                .to.contain(2);
        });
    });
});

• chai.plan(n) sets an internal assertionsPlanned variable to n and resets assertionsExecuted to 0
• each expect(...) call increments the assertionsExecuted counter
  • to make this work with the should interface we should increment in the Assertion constructor if a planned flag hasn't been set yet
• chai.verifyPlan() compared assertionsPlanned with assertionsExecuted and throws an error if they don't match
  • chai.verifyPlan() should also reset assertionsPlanned to 0 to make it work for following tests that don't use chai.plan(n)

[keithamus]

Hey @Turbo87 thanks for the issue.

We have discussed this before in #94. As per that discussion, it's a little tricky to pull off because the logic of what an "assertion" is with Chai's fluent syntax is very different to simpler test frameworks like that of Ava, Tape, Tap, Qunit et al.

Having said that, #94 is still open so we can come to a consensus, so feel free to continue the discussion there.

Closing this as its a dupe of #94