DEV Community

Easy functional programming techniques in TypeScript for everyone

Deepu K Sasidharan on August 30, 2019

Originally published at deepu.tech. There is a lot of hype around functional programming(FP) and a lot of cool kids are doing it but it is not a s...

Read full post

Amin • Aug 30 '19

Hi there, great article! Did you know that you can use Object.freeze to prevent mutating an array or an object?

const list = Object.freeze([])

list.push(1)
// Throws an error

Pragmatic Maciej • Aug 31 '19

It will couse runtime exception. So Object.freeze is not useful in terms of TS.

Amin • Aug 31 '19

Hi Maciej. You are absolutely right! That's why it is so good coupled with TypeScript which can prevent mutating a freezed object by refusing to transpile the file.

Pragmatic Maciej • Aug 31 '19

Still I see readonly as a tool for the job. Object.freeze is fully run-time thing and it is decoupled from type definition. Declaration of readonly is included into type definition, so it is explicit. I don't think run-time block is needed here.

Deepu K Sasidharan • Aug 30 '19

Yes, I thought of writing about it but then missed

stereobooster • Aug 30 '19 • Edited

Unfortunately, there are not many ways to strictly limit data mutation in TypeScript

type Foo = {
  bar: number;
  bas: number;
}
type FooReadonly = Readonly<Foo>;

const list = ["Apple", "Orange", "Banana", "Grape"] as const;

Mutation doesn't affect referential transparency - reference stays the same.

const test = {}
test.a = 1
test === test

Deepu K Sasidharan • Aug 31 '19

I have updated the post

Deepu K Sasidharan • Aug 30 '19

Yes, I missed that

Julian Garamendy • Aug 30 '19

Hi! It may be worth mentioning that
"TypeScript comes with a ReadonlyArray type that is the same as Array with all mutating methods removed, so you can make sure you don’t change your arrays after creation"

typescriptlang.org/docs/handbook/i...

Deepu K Sasidharan • Aug 30 '19

Oh yes, I completely forgot about those

Pragmatic Maciej • Aug 31 '19

functional data types such as Stacks, Maps and Queues.

Stack, Map and Queue are not data types related to FP, they have FP implementations, but still the sentence is wrong.

Deepu K Sasidharan • Sep 2 '19

I have rephrased it

Deepu K Sasidharan • Sep 1 '19

Sure they are not related to FP, what I meant was that they are more appropriate for the FP approach

Bob Myers • Sep 6 '19

Virtually everywhere you say "TypeScript", you seem to mean "JavaScript". It is not TypeScript that treats functions as first-class objects, or lazily evaluates boolean expressions. It's JavaScript. TypeScript is merely JavaScript with type annotations.

Deepu K Sasidharan • Sep 9 '19

Yes, and thats why I keep saying typeScript/JavaScript and also I made it clear in the beginning that TS is just a superset of JS

Jean-Michel 🕵🏻‍♂️ Fayard • Sep 9 '19

What kind of markdown do you use to link to the other articles at the top? It looks neat!

Deepu K Sasidharan • Sep 9 '19

Its the series feature in Dev, if you add series: my series name to the front matter of the post-it links them up that way