From Imperative to Functional: TypeScript Fetch Promise

From Imperative to Functional: Typescript Fetch Promise

This gist shows how more functional code is superior to imperative code to perform a simple fetch request to get a JSON value that models a tree data structure to be represented later by HTMLCanvasElement.

Both versions are correct and do the same, but the “functional” version still has some imperative style because JS/TS is not a functional language but the difference is clear again.

The difference between both snippets is the following:

async function fetchTree(path: string): Promise<TreeNode> {
  const onError = reason => showError({ reason, msg: 'Failed to fetch tree' });
  let tree = newTreeNode();

  try {
    const res = await fetch(path);

    if (res.ok) {
      tree = await res.json();
    else {
      onError(new Error(res.statusText));
  catch (reason) {
  return tree;
function fetchTree(path: string): Promise<TreeNode> {
  return fetch(path)
    .then(res => res.ok ? res : Promise.reject(res.statusText))
    .then(res => res.json())
    .catch(reason => {
      showError({ reason, msg: 'Failed to fetch tree' });
      return newTreeNode();
More Functional

The refactored code (a.k.a. “more functional”) is not functional, but it gets close. This is to avoid introducing functional abstractions like pipes, monads, etc., on top of JS/TS which is not a functional language but a mixed one as the underlying project is pretty short.

By noticing the:

👎🏻 return statement

👎🏻 ternary operator

👎🏻 error handling block

And lack of:

👉🏻 pipe operator

👉🏻 pattern matching

👉🏻 expressions1

Then we have reasons why the “functional” snippet doesn’t get even better.

The imperative version has the following visible problems, and I encourage you to reason the code snippets to figure them out:

  • Imperatively needs to add async to the function signature which leads to the “async-await hell” and more boilerplate.
  • Usage of mutable variable tree for returning the function value in order to avoid a multiple-return mess.
  • Usage of try-catch for error handling which has many disadvantages and can be replaced with sum types or monads like Rust does, so there’s no reason why we should keep using try-catch blocks in robust software development.
  • Usage of OOP which might enhance lower-level imperative code but is complete nonsense for high-level software (most projects) and yes, more boilerplate.
  • As I say below, it still has and needs mixed components (functional/declarative, OO, etc.) so if we could go fully functional (with better languages) why keep writing cheap code like that?
  • Has more formatting constraints, I always put the else branch on a new line (which gives more LoC) as it is factually the best way to format it.
  • It’s obviously quite prone to error and hard to read.
  • Even the Mozilla docs for Promise only show toy examples, and real life error handling gets worse with imperative code. Code on the internet like docs or tutorials almost always skip the status ok error handling and go directly to fail when parsing the JSON data. Who cares when code is imperative anyway?

Also, notice how throwing from a try-catch block is an antipattern as it becomes a goto23.

If TS had try-expressions (like Kotlin) the imperative version would get much better regarding correctness and style, but expressions are declarative so functional in the end 😋.

The more you make a program better, the more functional it gets, that’s because FP is the only/original programming paradigm there exists as per scientific concerns, and all other paradigms are just cheap workarounds.

Notice we can go “pure functional” but not “pure imperative” or “pure OO” so the workarounds are clearly the alternative non-functional paradigms.

Another good one I know a lot from experience is that the more I refactor code to improve it the more domain-specific it gets, and FP is clearly the natural way to go for DSLs.

I used to be a huge fan of Java as an OO approach (still have my good reasons) but I got to understand those “clever” ways of programming are just intellectual distractions.

So, my favorite phrase I use to teach others or tell my story is that the simplest designs are the best and FP is all about simplicity.

Mainstream languages like JavaScript, TypeScript, and Java don’t have good functional support, but we can still build better code regarding robustness and clearness by leveraging their available features and our computer science knowledge.

  1. Notice the stupid semicolons appear when the line of code is imperative, sounds familiar isn’t it? yes, Rust 🦀 

  2. You can learn plenty of these details from IntelliJ IDEA inspections 

  3. Understanding these details are what make you stand away from bad programmers and make you a competent one