[Retozi]

TypeScript is the one technology I would discard last of the entire web stack.

Using TypeScript instead of JavaScript is such a productivity boost for a team developing a sizeable single page application, any other technical decision (which Framework, tooling decisions) is almost irrelevant relative to this major improvement.

TypeScript has ALL the properties that should make adoption a no-brainer:

1) Increases developer productivity, especially over long term maintenance timeframes. (My non-scientific, ymmv estimate is at least 30%)

2) Reduces production exception rate significantly

3) Can be adopted gradually

4) Full JavaScript ecosystem still at disposal

People seem to massively underestimate the benefit of a typed language in a frontend codebase. At gunpoint, I would rather have the backend in a dynamically typed language and keep TypeScript for the frontend. Chances are that the distance between your database and the API is not far, so static typing is not that useful.

However, if you have decided for a client side rendered application, your app is stateful, dynamic and probably quite big. Static typing really helps to avoid mistakes and increases the ability to refactor quickly and correctly by order of magnitude.

There are too many hardly ever visited code branches, and tests are very costly to write for an application where users can click a lot of things.

[rubber_duck]

It's not even the fact that JS is dynamically typed it's that it's the worst of dynamic typing - magic strings, monkey-patching, retarded scoping rules coupled with callback pyramids all over the place, no proper modules - you just get no meaningful help from tooling as soon as your code base grows to even medium complexity. ES6 helps but TS brings all that + more.

Python IDEs like PyCharm offer far superior tooling even without static typing, and Clojure is so nice I don't usually even need the tools.

[r2dnb]

I get your point, but while I am a big advocate of typescript, I think that the topology of the javascript language is perfectly suited for front-end development.

There are a lot of impedance mismatches to deal with in the front end, (javascript to DOM, json to javascript, diversity of execution environments etc...), and I feel that when correctly used, dynamism the way javascript does it is a god-send.

All my core developments are in typescript, but in many situations it feels so magic to just do : myprop.undeclared = value and encapsulate the stuff in a knockout component written in typescript, while the situation would have caused a drama / major redesign in a back-end static language (C#, Java etc...). There are many valid scenarios (not resulting from a wrong OOP design) where this is very useful.

I'm actually amazed that people who designed Javascript got it so right. Static languages are great in the backend, typescript is great for the front-end and solved a real maintenance and Q&A issue by providing a level of typing and popularizing OOP in the front-end, but javacript is great for the web browser in general. Typescript is the piece that was missing, but this doesn't mean that javascript is broken, the elasticity of the language is a very powerful and elegant solution for the problems specific to the front-end.

[rubber_duck]

>myprop.unDeclared = value and encapsulate the stuff in a knockout component written in typescript

And when I need to read this code later on and figure out what the shape of myprop is what do I do ? When I come across a function in a different file that touches that "unDeclared" property how the hell do I figure out what type it is, where it was assigned, etc. How do I find all references to that if "unDeclared" is a common word or is used on multiple types as a property name ?

JavaScript is just a nightmare to maintain - sure it might be easier to bang out the code initially - but good luck finding someone who will maintain that stuff once the author moves on. Hell - good luck coming back to your code base a year later and finding your way around that. From my experience most of non-trivial JS ends up being write only.

[catnaroek]

> I think that the topology [emphasis mine] of the javascript language is perfectly suited for front-end development.

What exactly does “topology” mean in the context of programming languages? Are you trying to pull a Derrida (using fancy words without any actual meaning)?

> I'm actually amazed that people who designed Javascript got it so right.

Are you assuming that, even with a better language, the DOM's crappiness was inevitable, and thus we need a loosey-goosey language like JavaScript to deal with it? I doubt this is the case. Good languages make it easier to design good programs and APIs.

[jackmott]

3 & 4 in your list are so key. I was very skeptical of it until I started using it and realized that javascript is always valid typescript. So it never gets in your way. It only helps, and only if you want it to.

[nawitus]

I agree.

One benefit I've seen a lot is typing external services. A lot of code (both the frontend and the backend in say Node.js) tends to deal with calling external APIs, which would be dynamic in JavaScript and end up being quite loosely documented in the code.

With TypeScript, however, you can type in the JSON responses, which makes reading any code dealing with external services so much quicker. And these days with literal types you can even document that a particular property is either "magic_string_1" or "magic_string_2".

[wangchow]

Although, dynamic languages force extra incentive to unit-test the shit out of the codebase. Compilers certainly catch a lot of syntax mistakes and whatnot, but it also relaxes the requirement to add extra code coverage. The onus is more on the developer with dynamic typing but I'd take more-accurate code coverage over static types any day!

EDIT:

If you have static-typing and good unit-test coverage it's the best of both worlds I suppose. I tend to prefer duck-typing though which is sort of wedged in-between dynamic and static-typing in a sense. This however then becomes not a question of static vs. dynamic typing but of the language implementation of the type system.

In C++ for instance, the over-object-orientation is a pet peeve of mine when people define so many types and interfaces for this-and-that. I tend to prefer the "looseness" of template functions since you can pass whatever type in there so long as it contains the appropriate methods on the passed-in object. Such is static typing though so I'm digging myself into a hole from my original post :)

Now, a good static-analysis tool could catch issues on a dynamically-typed language in the same sense. Then I suppose the dynamic language becomes a pseudo-static language since a static-analysis algorithm is executed on the codebase. Since most build systems are set up to do: compile codebase; run static analysis to detect issues; run unit tests; run integration tests, it could theoretically be redundant step at compile-time doing static analysis when it's also done by an external tool. Just some stream-of-consciousness thoughts :)

[lubonay]

The only tests you have to discard when using a statically typed language are some of the tests validating your input data. I don't see how static typing leads to less test coverage...

[Retozi]

These are a lot of tedious to write tests in a frontend codebase that mainly renders a lot of state into HTML.

Additionally, these are tests that you will have to tediously adjust every refactor since HTML frontends and it's data APIs behind it tend to change often over the lifecycle of a product.

From my experience, "only those tests" save you hours and hours of programming time.

[catnaroek]

> The only tests you have to discard (...) are some of the tests validating your input data.

That's not true. You can safely discard anything that tests something you've proven. A good type system doesn't only prove things for you - it also makes it easier to manually prove what the type system can't prove.

[shados]

A good type system, yes. Something like TypeScript though? Only checks that variable types match. We're a far cry from ML/Elm/Scala/Haskell here.

So the only thing it helps is get rid of bad tests (eg: tests that check a function return any string...tests you shouldn't write even in a purely dynamic language).

[catnaroek]

> Something like TypeScript though? Only checks that variable types match.

TypeScript's type system is unsound, so you can't rely on the type checker alone to prove anything useful. However, manually proving things about TypeScript programs is still easier than doing so with JavaScript programs.

I don't believe it's practical to rely on type checkers to prove absolutely everything. There are mathematically proven upper bounds on how much you can automate reasoning. For instance, global type inference only works with first-order type languages. Higher-order anything flies out of the window, yet higher-order properties of programs need to be proven!

What we ought to do is find the optimal division of labor between automated and manual proving, always keeping in mind that the optimal case isn't being able to prove something. The optimal case is not having to prove it because it's obviously true. How do we design a programming language so that more interesting things are obviously true?

[catnaroek]

Sound type systems help you prove things. In other words, a type system is a tool that partially discharges part of the programmer's proof obligation. Of course, what you can't prove, you still need to test. This isn't news to anyone.

The problem is when the programmer doesn't know how much of his proof obligation is actually discharged by his use of the type system. (Hint: Not all of it!) And the solution is educating programmers, not replacing types with tests.

[skybrian]

And that's why I prefer static typing. All those trivial unit tests get in the way.

[shados]

You really shouldn't have a "trivial" test that a type system like TypeScript makes redundant, when doing pure JavaScript either.

If you have a function, eg: (a, b) => a + b The only thing TypeScript will help you enforce here is that a and b should be numbers, and the function should return a number.

Except, the test for this will be something such: "given 10 and 21, return 31". So you're testing the return type already. TypeScript will ensure that you never pass a string to a or b, for sure, but you should not have a test for that (unless you literred your code with type guards and wanted to test those. Don't do that).

Now, unless the function above is your final, public API (where TypeScript/Flow will have the biggest bang for the buck and have the most value add), you're probably calling that function somewhere. That somewhere will be tested too, and the full test harness will make sure strings never end up there.

You'll never actually need to write a test: "Make sure this is a string", and the only vulnerable area is the top level API, and that is a responsibility of the caller.

[wwweston]

> Chances are that the distance between your database and the API is not far, so static typing is not that useful.

For me, this mostly brings up the question of why the distance between data sources and front-end code apparently seems so big these days, and whether or not that's really an asset.