Introduction to TypeScript

by @leocaseiro

bit.ly/muses-code-ts

bit.ly/muses-code-ts-repo

What
is
TypeScript?

"Microsoft TypeScript is an open-source programming language developed and maintained by Microsoft. It is a strict syntactical superset of JavaScript, and adds optional static typing to the language."

wikipedia.org/wiki/Microsoft_TypeScript

=

+  types

What
is a
Transpiler?

Compiler vs Transpiler

 1100  1010  

1010  0101

0101  1100

compile to

transpile to

different level of abstration

same level of abstration

*.ts

*.js

JS data types (shapes)

  • null
  • undefined
  • boolean
  • number
  • string
  • Object
  • Array
  • Date*
  • Symbol (ES6)**

* TypeScript interface (object)

** not covered on this course

type of

typeof "text";
// string

null

undefined

let a = null;
console.log(a);
// null

typeof a;
// 'object' What?

There are two features of null you should understand:

 

  • null is an empty or non-existent value.
  • null must be assigned.

https://codeburst.io/javascript-null-vs-undefined-20f955215a2

Undefined most typically means a variable has been declared, but not defined.

 

You can also explicitly set a variable to equal undefined:

let b;
console.log(b);
// undefined

var d = {};
console.log(d.fake);
// undefined

var u = undefined;

boolean (binary)

true or false

1 or 0

let a = true;
console.log(a);
// true

let b = 1;
console.log(b);
// 1

console.log(a == b);
// true

console.log(a === b);
// false
let c = false;
console.log(c);
// false

let c = 0;
console.log(d);
// 0

console.log(c == d);
// true

console.log(c === d);
// false
let a = true;
typeof a;
// boolean

let b = 1;
typeof b;
// number

Not the same as truthy and falsy

number

integer, float (decimals)

let a = 123;
typeof a;
// number

let b = 12.3;
typeof b;
// number

console.log(a > b);
// true
let c = "123";
typeof c;
// string

let d = "12.3";
typeof d;
// string

console.log(c > d);
// true

string (text)

anything between quotes (", ', `)

let a = "John";
typeof a;
// string

let b = 'Doe';
typeof b;
// string

console.log(a + b);
// 'JohnDoe'

console.log(a + " " + b);
// 'John Doe'
let a = `John`;
typeof a;
// string

let b = `Doe`;
typeof b;
// string

console.log(`${a} ${b}`);
// 'John Doe'

typeof `123.00`;
// string

` (back-ticks)

object

let person = {
  firstName: "John",
  lastName: "Doe"
};

typeof person;
// object

typeof person.firstName;
// string
typeof window;
// object

typeof document;
// object

function / class*

function hello(name) {
  return `hello ${name}`;
}

typeof hello;
// function

class Calculator {
  sum = (a, b) => a + b
}

typeof Calculator;
// function

*syntax sugar for functions

var hello = (name) => {
  return `hello ${name}`;
}

typeof hello;
// function

array (collection)

let mixed = Array(1, 2, 'text', {name: 'John'})
// mixed = [1, 2, 'text', {name: 'John'}] // same



typeof mixed
// object

Array.isArray(mixed)
// true
typeof mixed[0];
// number

typeof mixed[2];
// string

typeof mixed[3];
// object

typeof mixed[4];
// undefined

Date (interface) 

let bday = new Date(1991, 11, 25);
typeof bday // Object

console.log(bday);
// Wed Dec 25 1991 00:00:00
// GMT+1100 (Australian Eastern Daylight Time)



new Date(year, monthIndex [, day [, hours [, minutes [, seconds [, milliseconds]]]]]);

JavaScript syntax

let user = "John Doe"

declaration

set

value

name

TypeScript syntax

let user: string = "John Doe"

declaration

set

value

name

annotation

type

Let's Play TS

typescriptlang.org/play

TS "or" syntax

let xyz: number | string = r;

or

type

type

TS Array<T> syntax

const numbers: Array<number> = [1,2,3,4]

const oddNumbers: number[] = [1,3,5,7,9];

<T> is a generic Type

Generic Types

  • any

  • <T>

type: any

interface Person {
  firstName: string;
}

let user: Person = {
  firstName: 'John'
}

user = 'John Doe';
// Type '"John Doe"' is not assignable to type 'Person'.
interface Person {
  firstName: string;
}

let user: any = {
  firstName: 'John'
}

user = 'John Doe';

--noImplicitAny = false

Generic <T>

function foo(arg: string): string {
    return arg;
}

foo('test'); // return string
foo(123); 
// Argument of type '123' is not assignable
// to parameter of type 'string'.
function foo<T>(arg: T): T {
    return arg;
}

foo('test'); // return string
foo(123); // return number
function foo<T>(list: T[]): T[] {
  console.log(list.length);

  return list;
}

Exercise 01

exercises/exercise-01-data-types.ts

5 minutes

npm start

JS function syntax

function sum (n1, n2) {
  const r = n1 + n2;
  return r;
}

declaration

output

return

arguments

name

TS function syntax

function sum (n1: number, n2: number): number {
  const r = n1 + n2;
  return r;
}

declaration

output

return

argument

name

argument

arg. type

arg. type

output type

TS arrow function syntax

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

declaration

output

return

argument

name

output type

argument type

TS void output  syntax

const setName = (n: string): void => {
  name = n;
}

no output

Exercise 02

exercises/exercise-02-function-types.ts

10 minutes

npm start

Enums

enum State {
  ACT = "Australian Capital Territory",
  NSW = "New South Wales",
  QLD = "Queensland",
  SA = "South Australia",
  TAS = "Tasmania",
  VIC = "Victoria",
  WA = "Western Australia"
}

const myState: State = State.NSW;
console.log(myState);
// New South Wales

Enums

enum Direction {
  Up, // 0
  Down, // 1
  Left, // 2
  Right // 3
}

var d = Direction.Up;
console.log(d); // 0
enum Direction {
  Up = 1, // 1
  Down, // 2
  Left, // 3
  Right // 4
}

var d = Direction.Up;
console.log(d); // 1

Classes

Constructor

class Person {
  firstName: string;
  lastName: string;

  constructor(first: string, last: string) {
    this.firstName = first;
    this.lastName = last;
  }
}

const user = new Person('John', 'Doe');
console.log(user.lastName);
// Doe
class Animal {
    move(distanceInMeters: number = 0) {
        console.log(`Animal moved ${distanceInMeters}m.`);
    }
}

class Dog extends Animal {
    bark() {
        console.log('Woof! Woof!');
    }
}

const dog = new Dog();
dog.bark();
dog.move(10);
dog.bark();

Class Inheritance (extends) 

class Animal {
  name: string;

  constructor(n: string) { this.name = n; }

  move(distanceInMeters: number = 0) {
    console.log(`moved ${distanceInMeters}m.`);
  }
}

class Horse extends Animal {
  constructor(name: string) {
    super(name);
  }

  move(distanceInMeters = 45) {
    console.log("Galloping...");
    super.move(distanceInMeters);
  }
}

Class Inheritance (extends, super) 

class Person {
  private firstName: string;

  lastName: number; // public

  constructor(first: string, last: string) { ... }
}

const user = new Person('John', 'Doe');

console.log(user.firstName);
// Property 'firstName' is private
// and only accessible within class 'Person'.

public and private

class Person {
    private age: number;
    protected name: string;
    constructor(n: string, a: number) { this.name = n; this.age = a; }
}

class Employee extends Person {
    constructor(name: string, age: number) {
        super(name, age);
    }

    getName() {
        return `Hello, my name is ${this.name}`;
    }

    getAge() {
        return `Hello, my age is ${this.age}`;
        // Property 'age' is private
        // and only accessible within class 'Person'.
    }
}

private vs protected

class Calendar {  
  private _month: number;  
  
  get month() {  
    if (this._month != null) {  
      return this._month;  
    }  
  }  
  
  set month(m: number) {  
    if (m > 0 && m < 13) {
      this._month = m;  
    }  
  }
}

Accessors (get, set)

typescriptlang.org/docs/handbook/classes.html#accessors

 strictPropertyInitialization = false

 typescriptlang.org/docs/handbook/release-notes/typescript-2-7.html#strict-class-initialization

Abstract Class / method

abstract class Animal {
    move(distanceInMeters: number = 0) {
        console.log(`Animal moved ${distanceInMeters}m.`);
    }

    abstract foo(n: string): string; // like an interface
}

class Dog extends Animal {
    bark() {
        console.log('Woof! Woof!');
    }

    foo(n: string) { return n; }
}

const dog = new Dog();
dog.move(5); // inherited
dog.foo('bar'); // class 'Dog' implements inherited
// abstract member 'foo' from class 'Animal'.

typescriptlang.org/docs/handbook/classes.html#abstract-classes

Interface

defining contracts

(like a custom type)

Interface Example

interface IPerson {
  age?: number;
  firstName: string;
  lastName: string;
}

let user: IPerson = {
  firstName: 'John',
  lastName: 'Doe'
};

user.age = 18;

TS interface syntax

interface Person {
  firstName: string;
  lastName: string;
}

interface

name

property

braces

type

semicolon

(no equal sign)

TS function interface syntax

interface SearchFunc {
    (source: string): boolean;
}

parentheses

TS interface usage

interface Person {
  firstName: string;
}

let user: Person = {
  firstName: 'John'
}

Interface - optional property

interface Person {
  firstName: string;
  age?: number;
}


const user: Person = {
  firstName: 'John'
}

user.age = 18;

optional property

(question mark)

optional property for functions

function setPerson(name: string, age?: number) {
  // ...
}

optional property

(question mark)

Interface - readonly property

interface Person {
  readonly firstName: string;
}


const user: Person = {
  firstName: 'John'
}

user.firstName = 'Jane'; // error*
// Cannot assign to 'firstName' because it is a read-only property.

(like const, but for properties)

Exercise 03

15 minutes

exercises/exercise-03-classes.ts

npm start

Some TS tricks

(variable as <T>)

let foo: string = 'John Doe';
foo = 0; 
// Type '0' is not assignable to type 'string'.

let foo: string = 'John Doe';
(foo as any) = 123;

// @ts-ignore

if (false) {
    // @ts-ignore
    console.log("hello");
}

.tsconfig

{
    "compilerOptions": {
        "outDir": "./built",
        "allowJs": true,
        "target": "es5"
    },
    "include": [
        "./src/**/*"
    ]
}

typescriptlang.org/docs/handbook/tsconfig-json.html

tsc --init

Declaration Types

*.d.ts

npmjs.com/~types
 

definitelytyped.org

 

github.com/DefinitelyTyped/DefinitelyTyped

TSLint

⚠️ TSLint will be deprecated some time in 2019. See this issue for more details: Roadmap: TSLint → ESLint. If you're interested in helping with the TSLint/ESLint migration, please check out our OSS Fellowship program

eslint.org/blog/2019/01/future-typescript-eslint

TSLint

TSLint + ESLint

typescript-eslint.io

Enables ESLint to support TypeScript

Code Analysis

+

Code Style Checker

Exercise 04

cd app;
yarn;
yarn start;

Install TypeScript

npm install typescript

# or

yarn add typescript

Install Declaration Types

npm install @types/node @types/react @types/react-dom

# or

yarn add @types/node @types/react @types/react-dom

Rename any file to be a TypeScript file.

*.js => *.ts

References

github.com/microsoft/TypeScript-React-Conversion-Guide

facebook.github.io/create-react-app/docs/adding-typescript

References / Images

 
 

What next?

 
 

Introduction to TypeScript - Muses Code JS

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