How can I make a promise reject for testing?

Question

I have a db.js set up to do all my database calls. This is an example of one of the functions that query the database.

db.getUserCount = function () {
return new Promise (function (resolve, reject) {
    db.users.count().then(function (result) {
        resolve (result);
    }, function(e) {
        reject (e);
    });
});

};

I am pretty new to JavaScript and testing. I have used mocha and chai to test that it resolves like this:

describe('getUserCount', function() {
    it('should be fulfilled when called', function() {
        return db.getUserCount().should.be.fulfilled; 
    });
});

How can I test the reject part of the promises. Do I have to use something like sinon or is there some simple way to make the promise fail?

It is a weird terminology to understand, the reject word, but I assume that you want to test for exceptions or errors. Testing is testing some condition so depending on the error condition or the exception case, modify your mocha test. — user2347763, Apr 25 '16 at 12:06
Avoid the [`Promise` constructor antipattern](http://stackoverflow.com/q/23803743/1048572) (in `getUserCount`)! — Bergi, Apr 25 '16 at 14:39
How do I go about that. Should I not be using the constructor or something? That is the only way I know how to make promises — nasoj1100, Apr 27 '16 at 13:09

score 0 · Answer 1 · answered Apr 25 '16 at 11:04

0

Make db.users.count() call to fail by either causing some change in database entry or in your api.

answered Apr 25 '16 at 11:04

Aditya Singh

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will this not make my test when it fulfills fail then – nasoj1100 Apr 25 '16 at 11:23
Yes it will. You should have 2 separate tests for testing success and fail paths – Aditya Singh Apr 25 '16 at 11:32

score 0 · Accepted Answer · answered Apr 25 '16 at 14:25

I ended up using sinon stubs so the other test would still pass.

describe('db.getUserCount rejection test', function() {
    sinon.stub(db, 'getUserCount').returns(Q.reject(new Error(errorMessage)));

    it('should be rejected when called', function() {
        return db.getUserCount().should.be.rejected;    
    });

    it.only('getUserCount responds with correct error message', function() {
        return db.getUserCount().catch(function(err) {
            expect(err.message).to.equal('Error could not connect to database');
        });

    });
});

How can I make a promise reject for testing?

2 Answers2

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