[cutler]

This killed it for me:

class Callback

.....

this.then = g => new Callback(callback =>

Can't we just keep OOP out of FP?

[homami]

Using class enables us to chain instances naturally in JS syntax, especially because JS lacks infix functions or custom operators. o1.then(f).then(g).run(cb)

[BinaryIdiot]

You can avoid the class syntax here and write a function to return the same if you prefer.