Usage Guide

Creating a Promise

There are a few ways to create a Promise. If you need to call functions that yield, you should use Promise.async:

local myFunction()
	return Promise.async(function(resolve, reject, onCancel)
		wait(1)
		resolve("Hello world!")
	end)
end

myFunction():andThen(print)

If you don't need to yield, you can use regular Promise.new:

local myFunction()
	return Promise.new(function(resolve, reject, onCancel)
		local connection

		someEvent:Connect(function(...)
			connection:Disconnect()
			resolve(...)
		end)

		onCancel(function()
			connection:Disconnect()
		end)
	end)
end

myFunction():andThen(print)

If you just want to wrap a single value in a Promise, you can use Promise.resolve:

local myFunction()
	return Promise.resolve("Hello world!")
end

myFunction():andThen(print)

If you already have a function that yields, and you want it to return a Promise instead, you can use Promise.promisify:

local function myYieldingFunction(waitTime, text)
	wait(waitTime)
	return text
end

local myFunction = Promise.promisify(myYieldingFunction)
myFunction(1.2, "Hello world!"):andThen(print):catch(function()
	warn("Oh no... goodbye world.")
end)

Rejection and errors

You must observe the result of a Promise, either with catch or finally, otherwise an unhandled Promise rejection warning will be printed to the console.

If an error occurs while executing the Promise body, the Promise will be rejected automatically with the error text and a trace back.

Chaining

One of the best parts about Promises is that they are chainable.

Every time you call andThen, catch, or finally, it returns a new Promise, which resolves with whatever value you return from the success or failure handlers, respectively.

somePromise:andThen(function(number)
  return number + 1
end):andThen(print)

You can also return a Promise from your handler, and it will be chained onto:

Promise.async(function(resolve)
	wait(1)
	resolve(1)
end):andThen(function(x)
	return Promise.async(function(resolve)
		wait(1)
		resolve(x + 1)
	end)
end):andThen(print) --> 2

You can also call :andThen multiple times on a single Promise to have multiple branches off of a single Promise.

Resolving a Promise with a Promise will be chained as well:

Promise.async(function(resolve)
	wait(1)
	resolve(Promise.async(function(resolve)
		wait(1)
		resolve(1)
	end))
end):andThen(print) --> 1

However, any value that is returned from the Promise executor (the function you pass into Promise.async) is discarded. Do not return values from the function executor.

Yielding in Promise executor

If you need to yield in the Promise executor, you must wrap your yielding code in a new thread to prevent your calling thread from yielding. The easiest way to do this is to use the Promise.async constructor instead of Promise.new:

Promise.async(function(resolve)
  wait(1)
  resolve()
end)

Promise.async uses Promise.new internally, except it allows yielding while Promise.new does not.

Promise.async attaches a one-time listener to the next RunService.Heartbeat event to fire off the rest of your Promise executor, ensuring it always waits at least one step.

The reason Promise.async includes this wait time is to ensure that your Promises have consistent timing. Otherwise, your Promise would run synchronously up to the first yield, and asynchronously afterwards. This can often lead to undesirable results. Additionally, Promise executors that only sometimes yield can lead to unexpected timing issues. Thus, we use Promise.async so there is always a guaranteed yield before execution.

Don't use regular spawn

Using spawn inside Promise.new might seem like a tempting alternative to Promise.async here, but you should never use it!

spawn (and wait, for that matter) do not resume threads at a consistent interval. If Roblox has resumed too many threads in a single Lua step, it will begin throttling and your thread that was meant to be resumed on the next frame could actually be resumed several seconds later. The unexpected delay caused by this behavior will cause cascading timing issues in your game and could lead to some potentially ugly bugs.

When to use Promise.new

In some cases, it is desirable for a Promise to execute completely synchronously. If you don't need to yield in your Promise executor, then you should use Promise.new.

For example, an example of a situation where it might be appropriate to use Promise.new is when resolving after an event is fired.

However, in some situations, Promise.resolve may be more appropriate.

Cancellation

Promises are cancellable, but abort semantics are optional. This means that you can cancel any Promise and it will never resolve or reject, even if the function is still working in the background. But you can optionally add a cancellation hook which allows you to abort ongoing operations with the third onCancel parameter given to your Promise executor.

If a Promise is already cancelled at the time of calling its onCancel hook, the hook will be called immediately.

It's good practice to add an onCancel hook to all of your asynchronous Promises unless it's impossible to abort an operation safely.

Even if you don't plan to directly cancel a particular Promise, chaining with other Promises can cause it to become automatically cancelled if no one cares about the value anymore.

If you attach a :andThen or :catch handler to a Promise after it's been cancelled, the chained Promise will be instantly rejected with the error "Promise is cancelled". This also applies to Promises that you pass to resolve. However, finally does not have this constraint.

If you cancel a Promise immediately after creating it without yielding in between, the fate of the Promise is dependent on if the Promise handler yields or not. If the Promise handler resolves without yielding, then the Promise will already be settled by the time you are able to cancel it, thus any consumers of the Promise will have already been called and cancellation is not possible.

If the Promise does yield, then cancelling it immediately will prevent its resolution. This is always the case when using Promise.async.

Attempting to cancel an already-settled Promise is ignored.

Cancellation propagation

When you cancel a Promise, the cancellation propagates up and down the Promise chain. Promises keep a list of other Promises that consume them (e.g. andThen).

When the upwards propagation encounters a Promise that no longer has any consumers, that Promise is cancelled as well. Note that it's impossible to cancel an already-settled Promise, so upwards propagation will stop when it reaches a settled Promise.

If a cancelled Promise has any consumers itself, those Promises are also cancelled.

Resolving a Promise with a Promise will cause the resolving Promise to become a consumer of the chained Promise, so if the chained Promise becomes cancelled then the resolving Promise will also become cancelled.

If you call resolve with a Promise within a Promise which is already cancelled, the passed Promise will be cancelled if it has no other consumers as an optimization.