however, enforcing it is somewhat difficult & there are still quite a bit lacking with working with plain objects or maps/sets.

As we all know, TypeScript is a super-set of JavaScript so at the end of the day your code is running in V8, JSCore or SpiderMonkey - depending on what browser the end user is using, as an interpreted language. It is also a loosely typed language with zero concept of immutability at the native runtime level.

And immutability in JavaScript, without native support that we could hopefully see in some hypothetical future version of EcmaScript, has the potential to impact runtime performance.

I work for a SaaS company that makes a B2B web application that has over 4 million lines of TypeScript code. It shouldn't surprise anyone to learn that we are pushing the browser to its limits and are learning a lot about scalability. One of my team-mates is a performance engineer who has code checked into Chrome and will often show us what our JavaScript code is doing in the V8 source code.

One expensive operation in JavaScript is cloning objects, which includes arrays in JavaScript. If you do that a lot.. if, say, you're using something like Redux or ngrx where immutability is a design goal and so you're cloning your application's runtime state object with each and every single state change, you are extremely de-optimized for performance depending on how much state you are holding onto.

And, for better or worse, there is a push towards making web applications as stateful as native desktop applications. Gone are the days where your servers can own your state and your clients just be "dumb" presentation and views. Businesses want full "offline mode." The relationship is shifting to one where your backends are becoming leaner .. in some cases being reduced to storage engines, while the bulk of your application's implementation happens in the client. Not because we engineers want to, but because the business goal necessitates it.

Then consider the spread operator, and how much you might see it in TypeScript code:

const foo = {

  ...bar, // clones bar, so your N-value of this simple expression is pegged to how large this object is

  newPropertyValue,

// same thing, clones original array in order to push a single item, because "immutability is good, because I was told it is"

const foo = [...array, newItem];

And then consider all of the "immutable" Array functions like .reduce(), .map(), .filter()

They're nice, syntactically ... I love them from a code maintenance and readability point of view. But I'm coming across "intermediate" web developers who don't know how to write a classic for-loop and will make an O(N) operation into an O(N^3) because they're chaining these together with no consideration for the performance impact.

And of course you can write performant code or non-performant code in any language. And I am the first to preach that you should write clean, easy to maintain code and then profile to discover your bottlenecks and optimize accordingly. But that doesn't change the fact that JavaScript has no native immutability and the way to write immutable JavaScript will put you in a position where performance is going to be worse overall because the tools you are forced to reach for, as matter of course, are themselves inherently de-optimized.

Generally immutability is also a programming style that comes with language constructs and efficient data structures.

Whereas 'read-only' (to me) is just a way of describing a variable or object.

`in` already implies the reference cannot be mutated, which is the bit that actually passes to the function. (Also the only reason you would need `in` and not just a normal function parameter for a class.) If you want to assert the function is given only a `record` there's no type constraint for that today, but you'd mostly only need such a type constraint if you are doing Reflection and Reflection would already tell you there are no public setters on any `record` you pass it.

Would your 'readonly' annotation dictate that at compile time ?

eg

class Test {

  private readonly string _testString {get;}


  public Test(string tstSrting) 
      => _testString = tstSrting ;

We may be going off topic though. As I understand objects in typescript/js are explicitly mutable as expected to be via the interpertor. But will try and play with it.

e.g., `let i = 0; i++;`

They seem to be only worried about modifying objects, not reassignment of variables.

1. This isn't an effort to make all variables `const`. It's an effort to make all objects immutable. You can still reassign any variable, just not mutate objects on the heap (by default)

2. Recursion still works ;)