chovy chovy - 4 years ago 112
Node.js Question

How do I properly test promises with mocha and chai?

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 Source

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
});

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

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