How do I properly test promises with mocha and chai?

Question

The following test is behaving oddly:

it('Should return the exchange rates for btc_ltc', function(done) {
    var pair = 'btc_ltc';

    shapeshift.getRate(pair)
        .then(function(data){
            expect(data.pair).to.equal(pair);
            expect(data.rate).to.have.length(400);
            done();
        })
        .catch(function(err){
            //this should really be `.catch` for a failed request, but
            //instead it looks like chai is picking this up when a test fails
            done(err);
        })
});

How should I properly handle a rejected promise (and test it)?

How should I properly handle a failed test (ie: expect(data.rate).to.have.length(400);?

Here is the implementation I'm testing:

var requestp = require('request-promise');
var shapeshift = module.exports = {};
var url = 'http://shapeshift.io';

shapeshift.getRate = function(pair){
    return requestp({
        url: url + '/rate/' + pair,
        json: true
    });
};

Answer 1

The easiest thing to do would be to use the built in promises support Mocha has in recent versions:

it('Should return the exchange rates for btc_ltc', function() { // no done
    var pair = 'btc_ltc';
    // note the return
    return shapeshift.getRate(pair).then(function(data){
        expect(data.pair).to.equal(pair);
        expect(data.rate).to.have.length(400);
    });// no catch, it'll figure it out since the promise is rejected
});

Or with modern Node and async/await:

it('Should return the exchange rates for btc_ltc', async () => { // no done
    const pair = 'btc_ltc';
    const data = await shapeshift.getRate(pair);
    expect(data.pair).to.equal(pair);
    expect(data.rate).to.have.length(400);
});

Since this approach is promises end to end it is easier to test and you won't have to think about the strange cases you're thinking about like the odd done() calls everywhere.

This is an advantage Mocha has over other libraries like Jasmine at the moment. You might also want to check Chai As Promised which would make it even easier (no .then) but personally I prefer the clarity and simplicity of the current version