 Courses from Jafar Husain

RxJS is a library for composing asynchronous and event-based programs by using observable sequences. 


rxjs

In this lesson we solidify our understanding of how to flatten collections. This is perhaps the most important skill when learning to program without loops. We will try our hand at flattening not just a two dimensional collection, but a three-dimensional collection. Later on it will become clear how these skills relate to asynchronous programming.

Advanced Flattening

***Note: Some of the RxJS syntax used in this course is outdated, but the concepts taught on arrays, async, and observables are still reliable and applicable.***

Async programming may seem daunting. How can we write programs that accept user input, run animations, and send server requests over the same period? How do we keep the code clear and concise? How do we gracefully propagate and handle asynchronous errors? How can we avoid memory leaks caused by dangling event handlers? JavaScript's loops and try/catch keywords are no help - they only work on synchronous functions.

Here's the good news: Asynchronous programming is much easier than it seems. The key is to think differently about events. It is possible to build most asynchronous programs using a handful of simple functions. We will learn why most JavaScript developers approach asynchronous programming the wrong way, and how to avoid these common mistakes. 

Asynchronous Programming: The End of The Loop

Armed with the `map` and `concatAll` functions, we can create fairly complex interactions in a simple way. We will use Observable to create a simple drag and drop example with basic DOM elements.

Simple drag and drop with Observables

Like an array, Observable has a map method that allows us to transform a sequence into a new Observable.

Using the map method with Observable

In this lesson we will get introduced to the `Observable` type. An Observable is a 'collection that arrives over time'. Observables can be used to model events, asynchronous requests, and animations. Observables can also be transformed, combined, and consumed using the Array methods we learned in the previous lessons. We can write powerful and expressive asynchronous programs using the few simple methods we've learned so far.

Introducing the Observable

In addition to flat Arrays, programmers must often deal with nested Arrays. For example let's say we have an Array of stock exchanges, each of which is represented by an array of all the stocks listed on that exchange. If we were looking for a stock that matched a certain criteria, we would first need to loop through all of the exchanges, and then all of the stocks within. 

In these situations, most developers would nest two loops. However in this lesson we will write a new Array function `concatAll` which will automatically flatten nested arrays buy one dimension. This will remove the need to ever use a nested loop to flatten a nested array.

Create an Array concatAll method

Both `map` and `filter` do not modify the array. Instead they return a new array of the results. Because both `map` and `filter` return Arrays, we can chain these functions together to build complex array transformations with very little code. Finally we can consume the newly created array using `forEach`. In this lesson, we will learn how to build nontrivial programs without using any loops at all.

Chaining the Array map and filter methods

One very common operation in programming is to iterate through an Array's contents, apply a test function to each item, and create a new array containing only those items the passed the test. For example, let's say you wanted to loop through an array of stocks and select only those with the price larger than a certain value. In this lesson we will demonstrate how to use the Array's `filter` method to easily perform this operation with less code than a loop would require.

The Array filter method

One very common operation in programming is to iterate through an Array's contents, apply a function to each item, and create a new array containing the results. For example, let's say you wanted to loop through an array of stock objects and select only the name for display on screen. In this lesson we will demonstrate how to use the Array's `map` method to easily perform this operation with less code than a loop would require.

The Array map method

Most JavaScript developers are familiar with the `for` loop. One of the most common uses of the `for` loop is to iterate through the items in an array. In this lesson, we will learn how to replace the `for` loop with the Array's `forEach` method - and shorten your code in the process.

The Array forEach method

Mock Service Worker is an API mocking library that allows you to write client-agnostic mocks and reuse them across any frameworks, tools, and environments.



Mocking WebSocket APIs with Mock Service Worker


While using anonymous functions might seem convenient, they can create difficulties when trying to identify performance bottlenecks during profiling. This lesson will demonstrate how named functions make it easier to pinpoint and address performance issues, ultimately leading to more efficient and debuggable code. Learning this practice will greatly improve your ability to locate and fix performance issues in complex projects.

Always Name Functions in JavaScript


When attempting to load the same module twice in JavaScript you'll hit a cache and code won't re-run. In scenarios where you actually do want to have state in your modules, you'll have to use a cache-busting technique by passing a query parameter to a dynamic import. This lesson walks through how to add a query parameter, how to add the appropriate types for loading dynamic modules in TypeScript, and when these techniques are valuable.

Import the Same JavaScript Module Multiple Times with Cache Busting



The include keyword in GitHub Actions empowers you to add custom variables to your matrix strategy, providing flexibility beyond standard configurations. In this video, we'll explore how include allows you to define specific combinations of variables, enhancing control over your workflow runs. You'll see how this feature enables you to tailor jobs for unique environments or scenarios not covered by the default matrix options. By leveraging include, you can efficiently manage complex build processes with greater precision. This technique helps you streamline your CI/CD pipeline, making it more adaptable to your project's evolving needs.

Use GitHub Actions Matrix Include for Complex Job Configurations

While running `zod` schema validation agains domain-rich objects, the error message saying that _a string is not a number_ might not be really useful.

Especially, when you need to provide a meaningful feeback to the user, e.g. after they have submitted a form with lots of data.

`zod` allows to customize messages on a very granular level. We can also extract repeatable customization to `zod` error maps.

Finally, in order to process an error message further (e.g. display it to the user), we need to handle the `zod` validation errors effectively. Especially, when a big object validation has failed and we could receive a handful of errors.

That's what we learn in this lesson. 🔥

Customize zod Error Messages and Handle zod Errors Effectively

customize zod validation messages, customize zod schema property messages, reuse error maps across different schemas, navigate through zod error objects

You might want to run `zod` schema checks in different styles.

Sometimes you prefer to conditionally throw the error - that's what `parse` does. But another time you'd prefer to receive a `boolean` property that determines whether the validation was successful or not - and perform further actions accordingly.

Additionally, the error-throwing API of `parse` works extremely well with [TypeScript assertion functions](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#assertion-functions). Their combination is just... perfect! 👌

That's what we learn in this lesson.

Use parse and safeParse Runtime Type-checks with zod

use zod parse function to conditionally throw ZodError, use zod safeParse to get a boolean value and run typescript type guard over success property

In this lesson we learn that TypeScript cannot guarantee correct shapes of responses. That's due to TS performing checks in compile-time, while the response will be availbale in runtime - when TS doesn't exist anymore.

In some situations, such as HTTP requests, runtime type checks such as `zod` are essential for providing quick feedback around data-related errors.

Understand where are the possible mistakes to make while using TypeScript with HTTP calls. Use `fetch` and `axios` along with `zod` schema checks.

Also, note that similar mistakes or mistakes could happen while using web storage (e.g. `localStorage`)

Validate HTTP API Response with zod, fetch and axios

use zod as an HTTP response validation tool along with fetch and axios in order to make sure that the response has expected data structure

In this lesson we learn that the `zod` schemas become the single source of truth.

We shall not update TypeScript types manually, since they are generated automatically from the schema.

Same with validation functions.

Whenever the schema changes, all the rest will update accordingly. It's an important mind shift.

Treat zod Schemas as Single Source of Truth

Zod Schemas are the single source of truth for both typescript types, and interfaces - and runtime validation functions. Schemas should be updated first.

`zod` is powerful, but writing the schemas manually for existing JSON objects, especially big ones, can be time-consuming.

But that's what `quicktype` is for! You can get 80% work done instantly! 🥳

Depending on your need - whether you want to automate the process, or do it just one time, you can:
- use an online app
- use an IDE extension
- run a CLI script
- generate a script to be executed within CI/CD environment/tooling

And this is what we learn in this lesson! 🔥🔥🔥

Get started with zod quickly!

Generate zod Schemas Automatically using Quicktype App, Extensions and Scripts

Use quicktype app, extension or scripts to automatically generate zod schemas from existing json files. Save time and get started with zod quickly!

`zod` is a powerful library allowing to run runtime type-checks.

It's complementary to TypeScript, which does type checks in compile time.

In this lesson we create few schemas that represent primitive type values, along with `zod` utilities which make these more precise. We also create object schemas and nest one inside another.

All schemas are being validated against example values. We analyze how to interpret error messages in case of failures.

Perform Runtime Type-checks on Primitives and Objects with zod

Create zod schemas to validate primitive type and object type values in runtime. Analyze the errors to see how a value doesn't correspond to schema expectations

Jafar Husain

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