What is TS anyway?

• Static Checking: ts detetcs errors in our code without running it
• Type Checking: find erroa, analyzes our code, available in development only, for development purposes

Installation

• Check out the ts docs
• Run npm i typescript --save-dev to install on this folder alone or npm i -g typescript to install globally
• Run tsc to test if it works

TypeScript Playground

• If you cannot install ts for any reason, use TypeScript Playground to try thing out or practice ts

Compile TS -> JS

• File with .ts ending
• To compile your ts code to js, run tsc filename

SECTION 1: TYPE ANNOTATION BASICS

• Assigning a basic type to variable by add :Type after variable name

• type need to be in all lowercase

  let myVar: type = value;
  
  const myVariable: string = 'This Is Awesome!!!';
  

Numbers & Booleans

• All lowercase as well, just number and boolean

const myNum: number = 7;
const myBoolean: boolean = true;

Compiling TS

• run tsc => we will see error shown in the terminal

Type Inference

//Type Inference
let tvShow = 'Criminal mind';
tvShow = 'CSI';
tvShow = false; //warning error type

Any Type

• When we don't annotate a variable's type AND we don't initialize it with a value, it has a type of "any"
• Allow you to use any types but it is sort of defeats the purpose of TS and types => use it sparingly

Delayed Initialization & Implicit Any

const movies = ['Titanic', 'CSI', 'Criminal Minds'];

let foundMovie;

for (let movie of movies) {
  if (movie === 'CSI') {
    foundMovie === 'CSI';
  }
}
foundMovie();
foundMovie = 1; //foundMovie was inference as type any, in this case we wanna specifically asign type for foundMovie without using type inference

SECTION 2: FUNCTIONS

• set type for your function's param
• without setting type, your param's type will be set default as any

function cal(num: number){
  return num * num
}
cal(6) // good
cal("hello") // ts will complain 

function sayHello(name: string){
  console.log("Oh hello ${name}")
}
sayHello("Ann") //good
sayHello(777) // ts will complain 

Default params (if we don't pass in any param)

• We can also work with default params like so:

//set name = there if when called function greeting() but does not provide any param
function greeting(name: string = "there"){
  console.log("Oh hello ${name}")
}
greeting() // "Oh hello there"
greeting("Ann") // "Oh hello Ann"

Return Type Annotation (return value type when call function)

• We can specify the type returned by a function by annotate the return type
• Add return type right after params. The doMath() return type is number const doMath = (x:number, y:number): number => {}