TypeScript: A Detailed Explanation of TypeScript's…

# Automatically Generate Default Configuration
tsc --init

# Generate a detailed configuration with comments
tsc --init --typescript

{
  "compilerOptions": {
    "target": "ES2020",
    "module": "ESNext",
    "strict": true,
    "outDir": "./dist",
    "rootDir": "./src"
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules", "dist"]
}

{
  "compilerOptions": {
    "target": "ES2020"
  }
}

{
  "compilerOptions": {
    "module": "ESNext"
  }
}

{
  "compilerOptions": {
    "moduleResolution": "node"
  }
}

{
  "compilerOptions": {
    "lib": ["ES2020", "DOM", "DOM.Iterable"]
  }
}

{
  "compilerOptions": {
    "outDir": "./dist",      // Compilation Output Directory
    "rootDir": "./src",      // Source Code Root Directory（Preserve the directory structure）
    "declaration": true,     // Generate .d.ts Statement
    "sourceMap": true        // Generate .js.map Source Code Mapping
  }
}

{
  "compilerOptions": {
    "strict": true
  }
}

// strictNullChecks: true
let name: string = null;    // ❌ null Cannot be assigned string
let name2: string | null = null;  // ✅ Explicit Declaration

// noImplicitAny: true
function greet(name) {      // ❌ The parameter is implicitly set to any
  return name;
}
function greet2(name: string) {  // ✅ Explicit Annotation
  return name;
}

// strictPropertyInitialization: true
class User {
  name: string;    // ❌ Property not initialized
  age: number = 0; // ✅ Has an initial value
}

// strict: false — the following code does not generate any errors, but it may crash during execution.
let name: string = null as any;    // Runtime name.toUpperCase() Breakdown
function greet(user) {             // user Implicit any
  return user.name;                // No type checking
}

// strict: true — captured at compile time
let name2: string | null = null;   // ✅ Must be explicitly declared null
function greet2(user: { name: string }) {  // ✅ Parameters must be labeled
  return user.name;
}

Compile error — strict mode catches null and implicit any issues

{
  "compilerOptions": {
    "baseUrl": ".",
    "paths": {
      "@/*": ["src/*"],
      "@utils/*": ["src/utils/*"],
      "@components/*": ["src/components/*"]
    }
  }
}

{
  "compilerOptions": {
    "resolveJsonModule": true,
    "esModuleInterop": true
  }
}

// Import Allowed JSON Documents
import config from "./config.json";
console.log(config.port);  // ✅

{
  "compilerOptions": {
    "allowJs": true,    // Allow compilation JS Documents
    "checkJs": false    // Do not check JS File Type（Compile Only）
  }
}

{
  "compilerOptions": {
    "noUnusedLocals": true,       // Error: Unused local variables
    "noUnusedParameters": true,   // Error: Unused function parameters
    "noImplicitReturns": true,    // Error: Function branch does not return a value
    "noFallthroughCasesInSwitch": true,  // Error: switch fallthrough
    "forceConsistentCasingInFileNames": true  // File names must be case-sensitive
  }
}

// noUnusedLocals: true
let unused = 42;    // ❌ Unused variables

// noUnusedParameters: true
function handler(event: Event) {  // ❌ event Unused
  console.log("Trigger");
}
// Fix: Use _ prefix tag
function handler2(_event: Event) {  // ✅ _ The prefix does not cause an error
  console.log("Trigger");
}

// noImplicitReturns: true
function getGrade(score: number): string {
  if (score >= 90) return "A";
  if (score >= 80) return "B";
  // ❌ Missing else Branched return
}

// noFallthroughCasesInSwitch: true
switch (action) {
  case "create":
    createItem();
    // ❌ Missing break——case Penetration
  case "update":
    updateItem();
    break;
}

{
  "compilerOptions": {
    "target": "ES2020",
    "module": "CommonJS",
    "moduleResolution": "node",
    "outDir": "./dist",
    "rootDir": "./src",
    "strict": true,
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,
    "resolveJsonModule": true,
    "declaration": true
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules", "dist", "**/*.test.ts"]
}

{
  "compilerOptions": {
    "target": "ES2020",
    "useDefineForClassFields": true,
    "lib": ["ES2020", "DOM", "DOM.Iterable"],
    "module": "ESNext",
    "skipLibCheck": true,
    "moduleResolution": "bundler",
    "allowImportingTsExtensions": true,
    "resolveJsonModule": true,
    "isolatedModules": true,
    "noEmit": true,
    "jsx": "react-jsx",
    "strict": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "noFallthroughCasesInSwitch": true,
    "baseUrl": ".",
    "paths": { "@/*": ["src/*"] }
  },
  "include": ["src"],
  "references": [{ "path": "./tsconfig.node.json" }]
}

{
  "compilerOptions": {
    "target": "ES2018",
    "module": "ESNext",
    "moduleResolution": "node",
    "declaration": true,
    "declarationDir": "./dist/types",
    "outDir": "./dist/esm",
    "rootDir": "./src",
    "strict": true,
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules", "dist", "**/*.test.ts"]
}

// The same source code compiles differently based on target/module settings
async function fetchData(): Promise<string> {
  let response = await Promise.resolve("hello");
  return response.toUpperCase();
}

export function greet(name: string): string {
  return `Hello, ${name}!`;
}

Compilation output varies by target/module — see explanations below

// strictNullChecks: true prevents common null bugs
interface User { name: string; email: string | null; }

function getDisplayName(user: User): string {
  // return user.email.toLowerCase(); // ❌ Object is possibly null
  return user.email ? user.email.toLowerCase() : user.name; // ✅
}

function findUser(id: number): User | null {
  return id > 0 ? { name: "Alice", email: "a@b.com" } : null;
}

let user = findUser(1);
// console.log(user.name); // ❌ user is possibly null
if (user) {
  console.log(user.name); // ✅ narrowed to User
}

Alice