Strongly Typed Error Handling in TypeScript

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TypeScript is a great language. TypeScript takes JavaScript and makes it actually good. If there’s one glaring weakness, it’s the inability to use strongly typed catch blocks. However, this is mostly due to a design flaw in the JavaScript language; in JavaScript, you can throw anything, not just Error types.


Consider the following, completely valid TypeScript code:

You can quite literally throw anything.

It’s easy to see why this can be a problem. One use-case where this is less than ideal is consuming a web API from TypeScript. It’s quite common for non-success HTTP status codes (500 Internal Server Error, 404 Not Found, etc.) to be thrown as an error by API consumer code.


Let’s consider an example using some utilities from AndcultureCode.JavaScript.Core and AndcultureCode.JavaScript.React, which wrap up API responses and errors in ResultRecord and ResultErrorRecord Immutable.js Record types.

You can see in this example that handling errors natively in TypeScript is… quite sloppy. The “maybe monad” common pattern to more generically handle errors and control flow. Basically, what we want to do is create an abstraction that can strongly type thrown errors to a specified type that you know is likely to be thrown. In our case, we want to be able to handle errors from a strongly typed ResultRecord with ResultErrorRecords inside it.

What if we could take the example above, and represent the same logic but with less code and strong typing in the catch block? In the following example, one of result or error will be non-null, but not both.

Automatic typecasting? Nice 😎

This pattern gives us a more functional approach to error handling, gives us strongly typed errors, and works really, really nicely when used in combination with React hooks. Let’s take a look at a simple React example using some infrastructure from AndcultureCode.JavaScript.Core, AndcultureCode.JavaScript.React and AndcultureCode.JavaScript.React.Components:

Strong typing is better than weak typing.

Clean, concise, and strongly typed error handling in just 46 lines of code, including the UI.


So how does this fancy-schmancy Do.try work under the hood? By adding an abstraction on top of regular old Promises. Let’s break it down.

First, let’s define some utility types we’re going to need:

Strongly typed handlers for strongly typed errors.

Next, let’s take a look at our constructor:

Private constructor? 🤔

That private constructor is no mistake. You’ll notice in the previous snippets, usage of this pattern starts with Do.try; that’s because try is a static factory method that returns an instance of Do. The private constructor can only be called internally to the class, by the try method. The implementation of try is very straightforward:

You’ll never know unless you… try.

The finally method is just as straightforward, with one important caveat:

Pay special attention to the return value.

Notice the return value, return this; This allows for method chaining, i.e. Do.try(workload).catch(catchHandler).finally(finallyHandler); In this code, catch and finally are both called on the same instance of Do which is returned from Do.try.

There’s also a getAwaiter method, which allows us to await for the result. All we need to do is return the internal promise.

This just allows us to await the result of the entire method chain.

Now let’s get to the interesting part; the catch method. Inside the catch method, we’re going to type guard the thrown object; if the thrown object is a ResultRecord instance, we cast it as such and pass it as the catch handler’s first argument; otherwise, it’s some unknown error, so we pass it as the catch handler’s second argument. We also need to cast the promise back to a Promise<TReturnVal> because of the return type of Promise.catch, but the promise is still a valid Promise<TReturnVal>.

We just need to cast the promise back to a Promise<TReturnVal> because of the return type of Promise.catch, but rest assured, the promise is still a valid Promise<TReturnVal>.

And there you have a basic implementation of a “maybe monad”. While the implementation here is an opinionated one, offering strongly typed error handling for ResultRecord errors, you could easily implement the same thing for virtually any type you want to use to wrap up your errors, just as long as you’re able to implement a type guard for it.

Taking It Further

I think strongly typed error handling speaks enough for itself, but we can take it even further. This pattern enables an extremely powerful utility, and I think it’s the strongest argument for using it: default behavior. We can extend our Do class to have a global configuration, allowing us to define default behavior which is applied to every instance of Do across the entire application.

All we need to do is add a static configuration mechanism, and implement a check for our configuration inside the constructor:

Default behavior is epic.

So what does it look like to apply default behavior? Let’s contrive an example.

We’re working on a large scale React application, and in order to aid debugging errors during development, we want to always log errors to the console in the development environment. Well, with the configuration mechanism we just added, it becomes trivially easy to add this default behavior. Just open up your index.ts app entrypoint and add the handler:

Log errors to the console by default.

You could use the same configuration mechanism to add default behavior to the try or finally portions of the call chain as well. Feel free to peruse the full implementation used in production here.

The syntax is quite nice to read and easy to understand at a glace, but with the added bonus of having strongly typed errors, and optional default behavior.

What do you think? Are you going to try “maybe monads” or the Do.try pattern in your next TypeScript project?

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