where to delete database and close connection after all tests using mocha

You can define a "root" after() hook before the 1st describe() to handle cleanup:

after(function (done) {    db.connection.db.dropDatabase(function () {        db.connection.close(function () {            done();        });    });});describe('User', ...);

Though, the error you're getting may be from the 3 asynchronous hooks that aren't informing Mocha to continue. These need to either call done() or skip the argument so they can be treated as synchronous:

describe('User', function(){    beforeEach(function(done){ // arg = asynchronous        done();    });    after(function(done){        done()    });    describe('#save()', function(){        beforeEach(function(){ // no arg = synchronous        });        // ...    });});

From the docs:

By adding a callback (usually named done) to it() Mocha will know that it should wait for completion.

I implemented it a bit different.

1. I removed all documents in the "before" hook - found it a lot faster than dropDatabase().

2. I used Promise.all() to make sure all documents were removed before exiting the hook.

   beforeEach(function (done) {    function clearDB() {        var promises = [            Model1.remove().exec(),            Model2.remove().exec(),            Model3.remove().exec()        ];        Promise.all(promises)            .then(function () {                done();            })    }    if (mongoose.connection.readyState === 0) {        mongoose.connect(config.dbUrl, function (err) {            if (err) {                throw err;            }            return clearDB();        });    } else {        return clearDB();    }});