As developers, we want to reduce fear of our code failing and increase trust in our code working well. Many developers working with JavaScript borrow useful ideas from functional programming and strongly-typed programming languages to reduce fear by transferring trust from developers to tools and the code. Ideas like optional types, functional transformations, and immutability can all help to write better JavaScript. When pulling these ideas together in JavaScript, however, they come with severe trade-offs, work together poorly, and ultimately fail in the goal of effectively transferring trust from developers to code and tools.

To illustrate this idea, let’s look at how data is handled in JavaScript from two perspectives: understanding the shape of data and changing data.

Fear and the shape of data

In a dynamic language like JavaScript, it can be hard to know what the shape of your data is. The default approach is to rely on convention. You trust other developers and other systems to give you correct data in agreed upon formats and to follow certain norms within the code base.

I like to call this the “pretend it’s what you want” approach. In high-trust environments, it can work well enough.

But then the fear creeps in. The code grows in complexity. You work with code from developers who follow different conventions. You receive data that you cannot control from upstream in erratic formats. You start seeing null pointer errors. Trust in the code breaks down, and questions about the data start to provoke anxiety rather than confidence.

What values does this data actually contain?

Can I delete these values without breaking things?

Can I pass in this data to this function?

You can see the fear in the code base. It looks like this:

This is defensive programming. It happens when you can no longer trust your own code to provide the data you expect at the appropriate times. Your beautiful code becomes cluttered with defensive checks, you lose readability, and the code becomes more brittle and harder to change. Fear grows, and it is harder and harder to trust that your code actually works.

Optional types: Pretend really hard

One way to stave off the fear is to introduce optional types via TypeScript or Flow. You receive a user and then proclaim joyously that it is of the User type, and henceforth shall be treated only as a User .

This is like pretending really hard. You’ve shifted your trust around. You still trust other systems to give you data in the correct shape. But within your code base, you trust the type that you’ve given to that data and that the compiler will complain if you use that data incorrectly. Instead of trusting developers to know the shape of data and use it appropriately, you’re trusting developers to write and maintain correct types, and you’re trusting the compiler to not lie about those types. More on that later.

Adding types to our example doesn’t solve the underlying problem. It improves trust within the code base by helping to ensure that data is used consistently, but it says nothing about data received from the outside world.

Validation: Trust but validate

In a low trust environment, you may need to introduce data validation at various points.

You could do this by hand, but the validation would be ad hoc, laborious, and error-prone. Or you could write JSON schema definitions and validate with ajv or the like to verify that the data matches your schema. This is less ad hoc and allows other uses like generating documentation, but is likely no less verbose or error-prone because you have to manually write out schemas like this:

Optional types + validation

Or you could introduce both types and validation. Types to stave off fear internally, and validation to be able to trust data from external sources.

To avoid writing essentially the same type definitions for both validation and optional types you can use the TypeScript or Flow compilers directly as libraries, or use another library like runtypes (TS), runtime-types (Flow), or typescript-json-schema (TS). After going through a few hoops you start feeling more trust in your data. But there are deeper issues here, which I will get to later.

Fear and changing data

What about when the data changes? By default in JavaScript data can change willy-nilly. For example, this function receives a document, and then changes the document to format a field properly and to include a new field.

But in this style, the flow is hard to follow, and fear starts to creep in. What if our data is used elsewhere? What if it was already changed elsewhere? What values do I have in my data at this point? How can I trust that the data I have at this point is the data I want at this point and will stay that way? This is a trivial example, but the problem becomes much worse with a large code base or a highly concurrent system.

You turn to optional types, but those types won’t save you. In TypeScript and Flow, both of these functions have the same type:

One of these does what you want; the other burns the city down. As far as these type systems are concerned, these functions do nothing.

Convention: Pretend immutability

So you write better JavaScript. You agree with your team, explicitly or implicitly, to write in an immutable style.

You favor const over var and duplicating values over mutation. You use let to indicate value references that change. You rediscover the ternary operator as a functional alternative to if statements, at least for short lines. You use functions to return new values instead of changing values. You use map , filter , reduce , and other functional constructs to create new data structures without changing the underlying data.

Immutability by convention is convenient, produces idiomatic JavaScript, and works well within the JavaScript ecosystem. But it relies heavily on both trust and discipline from developers. You trust developers to follow conventions like avoiding mutation or indicating clearly where mutation happens. You might want something stronger.

Libraries: Pretend really hard

You can shift the trust partly from other developers to tools by adopting libraries for data transformation and immutable data structures. You might start using a library like Ramda pervasively as a functional utility belt, or adopt lenses à la partial.lenses, monocle-ts, or the like.

One fundamental idea in these types of libraries is that the underlying data is treated as though it were immutable. It’s not – even Ramda only does shallow clones – but if the convention of immutable data is strong enough, then everyone can pretend it is. You may take a slight performance hit from copying data, but you gain some level of trust in the code. This works best if the use of the library and this convention is pervasive.

To enforce actual immutability and avoid the performance hit for changing data, you might also introduce immutable data structures via something like Immutable.js, seamless-immutable or Mori.

This makes the data itself actually immutable, in that only immutable ways to touch the data are exposed. But it only applies to data that is expressed within these data structures. As most of JavaScript relies on classic JavaScript data structures, you end up converting back and forth between the two a lot and you lose that trust whenever you have to use standard data structures.

Both of these approaches have limitations, but most importantly they clash hard with optional types.

Trusting JavaScript

The previous examples pulled out several tools for writing more effective JavaScript: optional types, functional transformations, and immutable data. But in JavaScript these tools come with some severe limitations, and they are hard to use together.

Optional types give a false sense of security

Optional types for JavaScript are optional by design, which means not everything is typed and you can’t trust that everything has a valid type. Flow is unsound and TypeScript is deliberately unsound, which means that in various cases the types are wrong and the compiler doesn’t care.

And optional types in JavaScript lie for other reasons. Some things in JavaScript are just hard or impossible to type out in TypeScript or Flow.

To type these out in TypeScript or Flow, you sacrifice on one or more principles:

Sacrifice type safety, the whole reason you use types: Type them out with any types, which allow any values and essentially disable the type checker for all values in the “path” of any . Sacrifice usefulness: Make the functions less general in order to provide more specific, accurate types. Sacrifice other developers’ time: Make the user of the function provide the correct types, as in

Then you add libraries into the mix, with their own type definitions with mixed levels of accuracy. This transfers some trust not to the developers of libraries, but to the developers of type definitions for libraries. Many of these libraries will contain any annotations, and calling those functions will quietly render your trust in types invalid. In Flow, type-checking can also be quietly disabled when a file is missing a @flow annotation.

You can work around this trust issue by adopting type annotations pervasively, disallowing both implicit and explicit any types, setting the linter to complain when files are not type-checked, and otherwise tightening up configurations.

But it’s like plugging holes in a leaky ship. The problem isn’t just that you can’t trust the types in your system, but that you think you can. You rely on the types to tell you when a change breaks something, but because they were quietly disabled by an any type, or by use of a library, or by a soundness issue, it doesn’t. Types in JavaScript are different from types in most other languages people use: They can’t be trusted in the same way.

Ultimately the strength of your types depends on the knowledge and belief of the team in applying them. If the team has a high level of belief and knowledge of types, they can encode a high level of trust into the system. But this is dependent on the team’s attention and discipline to maintain this level of trust, and fear can creep in and destroy that trust in many subtle ways.

Functional programming. Types. JavaScript. Pick two

Optional types and basic functional programming like maps and filters and reduces and so forth work alright together in JavaScript. It’s when you try to go further that you run into problems. Two examples:

Immutable.js is a persistent, immutable data structure library for JavaScript. It provides common data structures for JavaScript that do not rely on modifying the underlying data in-place. It has built-in type definitions for both TypeScript and Flow – go look at them. There are countless any annotations, which disable type-checking for those values. Then there are other types which pass the burden on to the user to provide the correct types. Essentially every time you use the library, you are either opting out of optional types or going to extra lengths to make the types work. This discourages functional programming.

Ramda is a functional utility library for JavaScript. One set of type definitions can be found here, along with this comment (emphasis added):

“Note: many of the functions in Ramda are still hard to properly type in Ramda, with issues mainly centered around partial application, currying, and composition, especially so in the presence of generics. And yes, those are probably why you’d be using Ramda in the first place, making these issues particularly problematic to type Ramda for TypeScript. A few links to issues at TS can be found below.”

Despite the impressive work of people like Giulio Canti, every time you choose even slightly more advanced functional programming concepts, like immutable data structures, function composition, or currying, you are essentially opting out of the type checker or going to extra lengths to make the types work. This discourages functional programming.

Why we can’t have nice things in JavaScript

Immutability works best when it is pervasive. But the JavaScript language and ecosystem is designed around mutable data, you can’t enforce immutability from a library, and optional types in JavaScript are not expressive enough to handle immutability as a library.

Types work best when they are pervasive. But types in JavaScript are optional by design and limit their expressiveness and utility by taking steep trade-offs to maintain compatibility with JavaScript.

Types, immutability, and functional programming can all support each other, just like they do in many languages. Types can be used to enforce immutability, even when the underlying data structures are mutable or the types don’t exist at runtime. Types can help developers connect the piping correctly when using functional composition or transforming data using lenses. Functional transformations can be easier to understand and maintain when you see the types. Functional transformations can be more efficient when you know the underlying data is immutable.

Learning to code with fear

So how do you learn to code with the fear? You write better JavaScript. You start with the base assumption that you can trust little in your code, and learn countless tricks to write more functional JavaScript and avoid the wartier parts of the language. You introduce type validation where necessary. You use immutable data, but only where you have a specific need or you enforce it by convention only. You adopt optional types where it makes sense, but abandon types where functional data handling or immutable data provide greater benefits. You use functional composition or lenses while knowingly opting out of type checking guarantees.

Or you change the game and just use PureScript. Or ReasonML, or Elm, or even ClojureScript. These exist today. Production software runs on them. They work with the JavaScript ecosystem, where necessary. And they provide a higher base level of trust in the code that you write and an environment where immutability, functional programming, and types (where applicable) work well and work together.

Adopting one of these languages is not going to solve all of your problems. It will introduce its own problems. But it might give you a higher level of basic trust in your code, and better tools to increase or decrease that trust as needed. In my next post, I discuss how these ideas play together in PureScript.

But in JavaScript, the fear is always with you.