Mocha supports any assertion library you want. You can take a look at how it deals with assertions here: http://mochajs.org/#assertions. I don't know which one you want to use.
Considering you are using Chai, which is pretty popular, here are some options:
Consider "foo" to be the target variable you want to test
var assert = chai.assert; assert(foo) // will pass for not null and not undefined // or assert(foo != null) // or assert.notEqual(foo, null);
In case you want to use
assert, you don't even need Chai. Just use it. Node supports it natively: https://nodejs.org/api/assert.html#assert_assert
var should = require('chai').should(); should.exist(foo); // will pass for not null and not undefined // or should.not.equal(foo, null);
var expect = chai.expect; expect(foo).to.not.equal(null); // or expect(foo).to.not.be.null;