It's a subclass of native ES6 Promise object, which evaluates only when then is called. It's written primarily for node.js, but should also run in browser.
const LazyPromise = require('lazy-promise');
let promise = new LazyPromise(function(resolve, reject)
{
resolve('ok');
});
promise.then(function(answer)
{
console.log('Everything is ' + answer);
});