Add onUnhandledRejection

Closes #54

lib/init.lua

@@ -222,6 +222,7 @@ local Promise = {
 	Status = makeEnum("Promise.Status", { "Started", "Resolved", "Rejected", "Cancelled" }),
 	_getTime = os.clock,
 	_timeEvent = game:GetService("RunService").Heartbeat,
+	_unhandledRejectionCallbacks = {},
 }
 Promise.prototype = {}
 Promise.__index = Promise.prototype
@@ -1809,6 +1810,10 @@ function Promise.prototype:_reject(...)
 			-- Build a reasonable message
 			local message = string.format("Unhandled Promise rejection:\n\n%s\n\n%s", err, self._source)
 
+			for _, callback in ipairs(Promise._unhandledRejectionCallbacks) do
+				task.spawn(callback, self, unpack(self._values, 1, self._valuesLength))
+			end
+
 			if Promise.TEST then
 				-- Don't spam output when we're running tests.
 				return
@@ -1986,4 +1991,25 @@ function Promise.fromEvent(event, predicate)
 	end)
 end
 
+--[=[
+	Registers a callback that runs when an unhandled rejection happens. An unhandled rejection happens when a Promise
+	is rejected, and the rejection is not observed with `:catch`.
+
+	The callback is called with the actual promise that rejected, followed by the rejection values.
+
+	@param callback (promise: Promise, ...: any) -- A callback that runs when an unhandled rejection happens.
+	@return () -> () -- Function that unregisters the `callback` when called
+]=]
+function Promise.onUnhandledRejection(callback)
+	table.insert(Promise._unhandledRejectionCallbacks, callback)
+
+	return function()
+		local index = table.find(Promise._unhandledRejectionCallbacks, callback)
+
+		if index then
+			table.remove(Promise._unhandledRejectionCallbacks, index)
+		end
+	end
+end
+
 return Promise

lib/init.spec.lua

@@ -35,6 +35,40 @@ return function()
 		end)
 	end)
 
+	describe("Unhandled rejection signal", function()
+		it("should call unhandled rejection callbacks", function()
+			local badPromise = Promise.new(function(_resolve, reject)
+				reject(1, 2)
+			end)
+
+			local callCount = 0
+
+			local function callback(promise, rejectionA, rejectionB)
+				callCount += 1
+
+				expect(promise).to.equal(badPromise)
+				expect(rejectionA).to.equal(1)
+				expect(rejectionB).to.equal(2)
+			end
+
+			local unregister = Promise.onUnhandledRejection(callback)
+
+			advanceTime()
+
+			expect(callCount).to.equal(1)
+
+			unregister()
+
+			Promise.new(function(_resolve, reject)
+				reject(3, 4)
+			end)
+
+			advanceTime()
+
+			expect(callCount).to.equal(1)
+		end)
+	end)
+
 	describe("Promise.new", function()
 		it("should instantiate with a callback", function()
 			local promise = Promise.new(function() end)