Get Started with Typescript

1 Get Started with Typescript

Any large and good apps written in Javascript will be eventually written in Typescript.

1-1 Types of Languages

• Dynamically Typed Language

  • Type check will be done at runtime
  • No need to assign data type to variables, very flexible
  • e.g. Python, Javascript etc.
  • Note Python has introduced the type hint

• Statically Typed Language

  • Type check happens during the compile phase
  • Need to declare the data type for variables
  • e.g. C++, Java

1-2 What is Typescript

• A Javascript superset
• Javascript that scales
• A Typed language
• Supporting ES6 through ES10 even ESnext
• Compatible with modern browsers, OS, servers
• Open Source

1-3 Why use Typescript

• Easy to understand
• Easy to prevent errors
• More efficient
• Completely Compatible with JS
• More and more popular frameworks are written in Typescript now

• Cons
  • Learning curve
  • Additional dev costs in the near term

1-4 Install Typescript

• Install Typescript

script

npm install -g typescript

• Check version

script

tsc -v

Version 3.8.3

• Install a certain version

script

npm i -g typescript@3.7.2

Check version again

script

Version 3.7.2

1-5 Hello World in Typescript

• Create a file hello.ts
• Edit the file

const hello = (name: string) => {
  return `hello ${name}`
}

hello('World')

• Compile the file

script

tsc hello.ts

Then the system will generate a js file

var hello = function (name) {
    return "hello " + name;
};
hello('World');

• Error during compiling

const hello = (name: string) => {
  return `hello ${name}`
}

hello('World')
hello(42)

System will report error if compiling

script

helloword.ts:6:7 - error TS2345: Argument of type '20' is not assignable to parameter of type 'string'.

6 hello(20)
        ~~
Found 1 error.

2 Data Type

2-1 Data types in Javascript

There are 7 primitive data types:
- Boolean
- Null
- Undefined
- Number
- BigInt (ES6)
- String
- Symbol (ES6)
And Object

2-2 Basic Data Types

Some basic data types:

let isDone: boolean = false

let age: number = 20
let binNumber: number = 0b1111

let firstN: string = "Isaac"
let msg: string = `Hello ${firstN}, age is ${age}`

let udf: undefined = undefined
let n: null = null
```    
note `undefine` and `null` are sub-type for `number`, `string` and `boolean`, we can define a `number` type `undefined`

Below is valid:

```typescript
let myNum: number = undefined

2-3 any Type and uniTypes

• For unknown types, we can use any
• Be cautious on any

let notSure: any = 4
notSure = "maybe it is a string"
notSure = true

• Unitypes: could be a combined types

let numberOrString: number | string = 234
numberOrString = "a string"

// below will be an error
// numberOrString = true

2-4 Array and Tuple

• Start with an array example
• Is limited only to the type defined

let arrOfNum: number[] = [1,2,3,4]
arrOfNum.push(5)

• arraylike objects in javascript
• In javascript there are some data types similar to arrays but are not actually arrays
• For example, arguments after defining a function

function test(){
  // arguments is similar to an array
  console.log(arguments)
  console.log(arguments.length)
}

• Tuples can contain different types

let user: [string, number] = ["this is a string", 20]

2-5 Interface

• Describe the object shape
• Class Abstraction
• Duck Typing

interface Person {
  name: string;
  age: number;
}

let isaac: Person = {
  name: "Isaac",
  age: 20
}

• Optional attribute

interface Person {
  name: string;
  // now age is optional
  age?: number;
}

let isaac: Person = {
  name: "Isaac",
}

• ReadOnly

interface Person {
  readonly id: number;
  name: string;
  age?: number;
}

let isaac: Person = {
  id: 1234,
  name: "Isaac",
}

// cannot change id, below will report error
// isaac.id = 3049

The difference between read-only and const

• read-only is applied to the attribute
• const is applied to the variable

2-6 Function

• Fixed arguments

function add(x:number, y:number):number {
  return x + y
}

let result = add(2,3)

• Optional arguments

function add(x:number, y:number, z?:number):number {
  if (typeof z === "number"){
    return x + y + z
  }
  else {
    return x + y
  }
}

let result = add(2,3, 5)
let result2 = add(3,5)

• Optional arguments cannot be placed after required arguments

// below will report error
function add(x:number, z?:number, y:number):number {
  if (typeof z === "number"){
    return x + y + z
  }
  else {
    return x + y
  }
}

• Function Expression

const add = function add(x:number, y:number, z?:number):number {
              if (typeof z === "number"){
                return x + y + z
              }
              else {
                return x + y
              }
            }

// below will report error
const add2: string = add

// correct
const add3:(x:number, y:number, z?:number) => number = add

• In the example above, add3 was automatically determined the type based on TS type inference

2-7 Class One

• Class defines the abstract attributes
• Object: an instance of class
• Four Principles of OOP:
  • Encapsulation: only expose interfaces
  • Inheritance: child class inherits parent class attributes, potentially with their own attributes
  • Abstraction: abstract attributes
  • Polymorphism: Different inherited classes might have different implementations for the same method

class Animal {
  name: string;
  constructor(name:string) {
    this.name = name
  }
  run(){
    return `${this.name} is running`
  }
}

const python = new Animal("three")

console.log(python.run());

class Dog extends Animal {
  bark(){
    return `${this.name} is barking`
  }
}

const lasse = new Dog("lasse")
console.log(lasse.run());
console.log(lasse.bark());

class Cat extends Animal {
  constructor(name) {
    super(name);
    console.log(this.name);
  }
  run(){
    return `Cat ` + super.run()
  }
}

const cat = new Cat("Garfield")

console.log(cat.run());
```        

## 2-8 Class Two

- There are three types of attributes
    - public
    - private
    - protected
    
- public: we can change the attribute and 

```typescript
class Animal {
  public name: string;
  constructor(name:string) {
    this.name = name
  }
  run(){
    return `${this.name} is running`
  }
}

const python = new Animal("three")

console.log(python.run());
console.log(python.name)
python.name = "3.8"
console.log(python.name)
```  

- private: for some attributes or methods inaccessible from the outsiders, including child class

```typescript
class Animal {
  private name: string;
  constructor(name:string) {
    this.name = name
  }
  run(){
    return `${this.name} is running`
  }
}

• protected: child class can access the protected attributes or methods

class Animal {
  protected name: string;
  constructor(name:string) {
    this.name = name
  }
  run(){
    return `${this.name} is running`
  }
}

• readonly: can only read but not edit

class Animal {
  readonly name: string;
  constructor(name:string) {
    this.name = name
  }
  run(){
    return `${this.name} is running`
  }
}

• static attributes and methods: can be accessed without instantiation

class Animal2 {
  readonly name: string;
  static categories: string[] = ["mammal", "bird", "others"]
  static isAnimal(a) {
    return a instanceof Animal2
  }
  constructor(name:string) {
    this.name = name
  }
  run(){
    return `${this.name} is running`
  }
}

const snake = new Animal2("lily")

console.log(Animal2.categories)
console.log(Animal2.isAnimal(cat))
console.log(Animal2.isAnimal(snake))

2-9 Interface and Implements

• Use interface and implements to abstract class attributes and methods

interface Radio {
  switchRadio(trigger: boolean): void;
}

interface Battery {
  checkBatteryStatus(): number;
}

interface RadioWithBattery extends Radio{
  checkBatteryStatus(): number;
}

class Car implements Radio{
  switchRadio(){
  
  }
}

class Cellphone implements Radio, Battery{
  switchRadio(){
  
  }

  checkBatteryStatus():number {
    return 0
  }
}

class Cellphone2 implements RadioWithBattery{
  switchRadio(){
  
  }

  checkBatteryStatus():number {
    return 0
  }
}

2-10 Enumerates

enum Direction {
  Up = 10,
  Down,
  Left,
  Right
}

console.log(Direction.Left)
console.log(Direction[2])

2-11 Generics One

• Generics: don’t specify the data type when defining a function or interface

function echo<T>(arg:T):T {
  return arg
}

const res = echo("str")

• Multiple values

function swap<T, U>(tuple:[T,U]):[U,T] {
  return [tuple[1], tuple[0]]
}

const result = swap([1, "string"])
console.log(result)

2-12 Generics Two

• Restricted Generics

interface IWithLength {
  length: number,
}

function echoWithLength<T extends IWithLength>(arg: T): T {
  console.log(arg.length)
  return arg
}

const str = echoWithLength("string")
console.log(str);

const obj = echoWithLength({length: 10, name: "Isaac"})
console.log(obj);

2-13 Generics Three

• Generics on Class

class Queue<T> {
  private data = [];
  enqueue(item: T){
    return this.data.push(item)
  }
  dequeue(): T {
    return this.data.shift()
  }
}

const queue = new Queue<number>()
queue.enqueue(1)
queue.enqueue(2)
console.log(queue.dequeue().toFixed())
console.log(queue.dequeue().toFixed())

const queue2 = new Queue<string>()
queue2.enqueue("str")
queue2.enqueue("num")
console.log(queue2.dequeue().length)
console.log(queue2.dequeue().length);

• Generics on Interface

interface KeyPair<T, U> {
  key: T;
  value: U;
}

let kp1: KeyPair<string,number> = {key:"A", value:2}
let kp2: KeyPair<number,string> = {key: 1, value:"str"}

let arr:number[]=[1,2,3]
let arr2:Array<number> = [1,2,3]

• Interface with function

interface IPlus {
  (a: number, b:number) : number
}
function plus(a:number, b:number) :number {
  return a + b
}

const res:IPlus = plus

• Generic Interface with Function

interface IPlus<T> {
  (a: T, b:T) : T
}
function plus(a:number, b:number) :number {
  return a + b
}
function connect(a:string, b:string):string {
  return a + b
}
const res:IPlus<number> = plus
const res2:IPlus<string> = connect

console.log(res(9,8))
console.log(res2("hello", "world"))

2-14 Type aliases

• to define a function

function sum(x:number, y:number):number {
  return x + y
}

const sum2: (x:number, y:number) => number = sum

• alternatively, can use type aliases

function sum(x:number, y:number):number {
  return x + y
}

type PlusType = (x:number, y:number) => number

const sum2:PlusType = sum

• join type

type NameResolver = () => string
type NameOrResolver = string | NameOrResolver
function getName(n:NameOrResolver):string {
  if (typeof n === "string") {
    return n
  } else {
    return n()
  }
}

• type assertion

function getLength(input:number|string) :number {
  const str = input as String
  if(str.length){
    return str.length
  } else {
    const num = input as number
    return num.toString().length
  }
}

an easier way to write above

function getLength(input:string|number):number {
  if((<string>input).length){
    return (<string>input).length
  } else {
    return input.toString().length
  }
}

2-15 Declare File

• Declare var to use 3rd party libraries

declare var jQuery: (selector: string) => any

• Save the above in a .d.ts file, e.g. jQuery.d.ts

• tsconfig file

{
  "include": ["**/*"]
}