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const getKeys = (obj: unknown) => {
  return Object.keys(obj as object) // <- cast the type to be object
}

const getObject = (): object | undefined => {
  // some code that return EITHER an object OR undefined 💣💣💣
}

getKeys(getObject()) // <- 💣💥 when getObject() returns undefined

vinhlee95

// Anything is assignable to unknown
const foo: unknown = 1 // <- we can assign a number to a unknown var
const bar: unknown = true // <- same with other primitive types such as boolean

// But unknown isn’t assignable to anything but itself
const myString: string = foo // <- ⛔️⛔️⛔️ unknown type is not assignable to string type
const isGreaterThan10 = (val: unknown) => val > 10 // <- ⛔️⛔️⛔️ because val has unknown type

vinhlee95

The unknown type in TypeScript

The unknown mistery

When I first onboarded TypeScript world from JavaScript, one of the most confusing thing is the unknown type. Remember when your coworkers write something like:

const foo = () => bar as unknown as any // <- 🤷🏻‍♂️🤷🏻‍♂️🤷🏻‍♂️

Let's take a look at TypeScript official documentation:

vinhlee95

Resources:
  MyInstance:
    Type: AWS::EC2::Instance
    Properties:
      AvailabilityZone: "eu-north-1a"
      ImageId: "ami-0c5254b956817b326"
      InstanceType: "t3.micro"
      KeyName: "ec2-general"
      SecurityGroups:
        - !Ref HTTPSecurityGroup

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const state = {
  idle: "idle",
  fetching: "fetching",
  success: "success",
  error: "error"
};

const todoActions = {
  fetchTodos: "fetchTodos",
  cancelFetching: "cancelFetching",

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functions:
  todos:
    handler: src/todos/todos.handler
    events:
      http: GET /todos

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import {from} from 'rxjs/operators'

const fetchCakesPromise = fetch(getCakesEndpoint, {method: 'GET'}).then(res => res.data.cakes)

// Get cakes stream creation 🌈
const cakeSource = from(fetchCakesPromise)

// Subscribe cake data and consume received cakes 🍽
cakeSource.subscribe(console.log)

vinhlee95

import { Observable } from 'rxjs'

const observable = new Observable(observer => {
	const array = [1,2,3,4,5]
	array.forEach(number => {
		if(!isOdd(number)) {
			observer.error('Invalid even number')
		}

		observer.next(number)

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keyPressSources.pipe(
	// Input handlers to take input value for searching
	switchMap(value => {
		return from(fetchCakesByName(value)).pipe(
			// Only make an API request for searching after 0.5s since user stopped pressing a key
			debounce(0.5),
			// Retry 3 times if an API request fails
			retry(3),
			// Cancel the search when press Esc or clear input
			takeUntil(value => value === 'Esc' || value === ''),

vinhlee95

MySQL

Getting started:

• http://www.sqlteaching.com/
• https://www.codecademy.com/courses/learn-sql

Related tutorials:

• MySQL-CLI
• Analyzing Business Metrics