chapter-2-notes.md

Chapter 2 Notes

TypeScript Compiler Process

1. (TSC) | TypeScript Source Code → TypeScript AST
2. (TSC) | AST is checked by Type Checker
3. (TSC) | TypeScript AST → JavaScript Source
4. (JS Runtime) | JavaScript Source → JavaScript AST
5. (JS Runtime) | AST → bytecode
6. (JS Runtime) | Bytecode is evaluated by runtime

Definitions

• Type System: a set of rules that a typechecker uses to assign types to
  your program

Two kinds of Type Systems

• Explicit Syntax: it is required to tell the compiler what type everything is
• Inference: Allow the compiler to infer the type of everything automatically

TypeScript is inspired by both.

Examples of Explicit TypeScript Types

let a: number = 1; // a is a number
let b: string = 'hello'; // b is a string
let c: boolean[] = [true, false]; // c is an array of booleans

Examples where TypeScript will Infer Types

let a = 1; // a is a number
let b = 'hello'; // b is a string
let c = [true, false]; // c is an array of booleans

In general, it is good style to let TypeScript inver as many types as it can
for you, keeping explicitly typed code to a minimum

Initialize a TypeScript Config

{
    "compilerOptions": {
        "target": "ESNext",
        "module": "commonjs",
        "esModuleInterop": true,
        "forceConsistentCasingInFileNames": true,
        "strict": true,
        "skipLibCheck": true,
        "lib": ["es"],
        "outDir": "dist",
        "sourceMap": true,
        "removeComments": false
    },
    "include": ["src/**/*"]
}

ESLint w/ TypeScript (instead of TSLint)

• https://typescript-eslint.io/getting-started/