Anurag Hazra anuraghazra

ListBox

Listbox seems to have all of the constraints in one component. Making a decision here is likely to be easily expanded to the rest of the components. It has all the tricky parts we're dealing with right now:

Index based focus management
Children needing their index to render
Parent/sibling component needing props from a sibling
Case for SSR support

any: magic, ill-behaved type that acts like a combination of never (the proper [bottom type]) and unknown (the proper [top type])
- Anything except never is assignable to any, and any is assignable to anything at all.
- Identities: any & AnyTypeExpression = any, any | AnyTypeExpression = any
- Key TypeScript feature that allows for [gradual typing].
unknown: proper, well-behaved [top type]
- Anything at all is assignable to unknown. unknown is only assignable to itself (unknown) and any.
- Identities: unknown & AnyTypeExpression = AnyTypeExpression, unknown | AnyTypeExpression = unknown

Prefer over any whenever possible. Anywhere in well-typed code you're tempted to use any, you probably want unknown.

Electives

This group is great for people building shared component libraries:

Compound Components

The foundation of all reusable components in React. Once you understand this concept, your components will be able to work in UIs you aren't even thinking about yet.

Why using the `children` prop makes `React.memo()` not work

I've recently ran into a pitfall of [React.memo()][memo] that seems generally overlooked; skimming over the top results in Google just finds it mentioned in passing in a [React issue][regit], but not in the [FAQ] or API [overview][react-api], and not in the articles that set out to explain React.memo() (at least the ones I looked at). The issue is specifically that nesting children defeats memoization, unless the children are just plain text. To give a simplified code example:

const Memoized = React.memo(({ children }) => (<div>{children}</div>));
// Won't ever re-render
<Memoized>bar</Memoized>
// Will re-render every time; the memoization does nothing

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

"Art prior" to React:

Martin Fowler: InversionOfControl (2005-Jun-26)

Inversion of Control is a key part of what makes a framework different to a library. A library is essentially a set of functions that you can call, these days usually organized into classes. Each call does some work and returns control to the client.

A framework embodies some abstract design, with more behavior built in. In order to use it you need to insert your behavior into various places in the framework either by subclassing or by plugging in your own classes. The framework's code then calls your code at these points.

The litmus test:

If your code calls it, it's a library.

	/* eslint-disable no-unused-vars */
	/* eslint-disable no-else-return */

	// JSX constructor, similar to createElement()
	export const h = (type, props, ...children) => {
	return {
	type,
	// Props will be an object for components and DOM nodes, but a string for
	// text nodes
	props,

	const https = require("https");

	exports.handler = (req, ctx, callback) => {
	const event = JSON.parse(req.body);
	console.log("Event:", event);
	let response = {
	statusCode: 200,
	body: "ok"
	};

	// a typescript implement of hindley-milner type inference
	// reference http://smallshire.org.uk/sufficientlysmall/2010/04/11/a-hindley-milner-type-inference-implementation-in-python/

	/// <reference path="./lib.es6.d.ts" />

	// ...
	interface AstNode {
	}

	class Id implements AstNode {