There are custom Promise implementations (for reasons), such as bluebird.js. If you're supporting legacy browsers, there will be no standard Promise object. So the simplest way to check for the Promise contract is the code posted. But yes, in an ideal world, one would be able to just do `promise instanceof Promise`.
These cases should really be handled at the compilation/transpilation level, since there is one, and users should just write latest generation JavaScript without these concerns.
