promise.then.js

_then = Promise.prototype.then
// [Function: then]

Promise.prototype.then = function(){ console.log('then!'); return _then.apply(this, arguments);  }
// [Function (anonymous)]

p = new Promise(resolve => resolve())
// Promise { undefined }

p.then()
// then!
// Promise { <pending> }

(async function () { await Promise.resolve(1) })()
// Promise { <pending> }

(async function () { await new Promise(resolve => resolve()) })()
// Promise { <pending> }

thennable.js

x = { then: (fn) => { console.log('thennable!'); fn()  } }
// { then: [Function: then] }

(async function () { await x })()
// Promise { <pending> }
// thennable!

node v13.3.0