Rudomanenko Volodymyr layerok

Advanced memoization and effects in React

Memoization is a somewhat fraught topic in the React world, meaning that it's easy to go wrong with it, for example, by [making memo() do nothing][memo-pitfall] by passing in children to a component. The general advice is to avoid memoization until the profiler tells you to optimize, but not all use cases are general, and even in the general use case you can find tricky nuances.

Discussing this topic requires some groundwork about the technical terms, and I'm placing these in once place so that it's easy to skim and skip over:

Memoization means caching the output based on the input; in the case of functions, it means caching the return value based on the arguments.
Values and references are unfortunately overloaded terms that can refer to the low-level implementation details of assignments in a language like C++, for example, or to memory

I heard some points of criticism to how React deals with reactivity and it's focus on "purity". It's interesting because there are really two approaches evolving. There's a mutable + change tracking approach and there's an immutability + referential equality testing approach. It's difficult to mix and match them when you build new features on top. So that's why React has been pushing a bit harder on immutability lately to be able to build on top of it. Both have various tradeoffs but others are doing good research in other areas, so we've decided to focus on this direction and see where it leads us.

I did want to address a few points that I didn't see get enough consideration around the tradeoffs. So here's a small brain dump.

"Compiled output results in smaller apps" - E.g. Svelte apps start smaller but the compiler output is 3-4x larger per component than the equivalent VDOM approach. This is mostly due to the code that is usually shared in the VDOM "VM" needs to be inlined into each component. The tr

We want to upload file to a server with POST HTTP request. We will use curl functions.


// data fields for POST request
$fields = array("f1"=>"value1", "another_field2"=>"anothervalue");

// files to upload
$filenames = array("/tmp/1.jpg", "/tmp/2.png");

Promises, under the hood

Everyone knows that to create a new Promise, you need to define it this way:

  new Promise((resolve, reject) => {
    ...
    resolve(someValue)
  })

You pass a callback that defines the specific behavior of your promise.

	export default function SessionsList() {
	const sessionsQuery = useSessionsListQuery();

	return (
	<PageLayout title="My sessions">
	{matchQueryStatus(sessionsQuery, {
	Loading: (
	<>
	<Skeleton height={70} mt={6} />
	<Skeleton height={70} mt={6} />

	import type { GetState, PartialState, SetState, State } from "zustand";

	const noop = (..._: unknown[]): void => {
	/* noop */
	};

	export type Setter<T extends State> = (
	s: Partial<T> \| ((prev: T) => Partial<T>),
	replace?: boolean
	) => void;

	/**
	* Postman Pre-Request script to append CSRF token in header for POST requests in Laravel
	* Sanctum authenticated SPA. Requires active environment with {{url}} variable defined
	* for main app domain.
	*
	* Postman Interceptor allows appending cookies from browser, but Laravel CSRF middleware
	* only validates CSRF in headers or in _token form field, not in cookies. Axios automatically
	* appends the CSRF from cookie to headers, but Postman cannot access intercepted cookies
	* and use them, so we have to do one pre-request to get the CSRF token, store it
	* in environment so it can be reused, and then append it to headers.

	#!/bin/sh

	########################################################################################
	# I do not need to use the ngrok subdomains...
	# start ngrok for my local box (vagrant homestead) in th background
	# get the cname from command line for the ngrok alias
	# update my subdomain cname to point at new ngrok name
	# use it!!!
	########################################################################################
	# requires ngrok and jq installed.

	let cache = new Map();
	let pending = new Map();

	function fetchTextSync(url) {
	if (cache.has(url)) {
	return cache.get(url);
	}
	if (pending.has(url)) {
	throw pending.get(url);
	}

	import styled, { css } from 'emotion/react'
	import { space, width, fontSize, color, responsiveStyle } from 'styled-system'


	export const display = responsiveStyle('display')
	export const flex = responsiveStyle('flex')
	export const order = responsiveStyle('order')
	const wrap = responsiveStyle('flex-wrap', 'wrap', 'wrap')
	const direction = responsiveStyle('flexDirection', 'direction')
	const align = responsiveStyle('alignItems', 'align')