typescript-type-system

TypeScript Type System

Master TypeScript's type system features to write type-safe code. This skill focuses exclusively on TypeScript language capabilities.

TypeScript Compiler

# Type check without emitting files
tsc --noEmit

# Type check with specific config
tsc --noEmit -p tsconfig.json

# Show compiler version
tsc --version

# Watch mode for development
tsc --noEmit --watch

Strict Mode Configuration

tsconfig.json strict mode options:

{
  "compilerOptions": {
    "strict": true,                           // Enables all strict
    "noImplicitAny": true,                    // Error on 'any'
    "strictNullChecks": true,                 // null must be explicit
    "strictFunctionTypes": true,              // Stricter function types
    "strictBindCallApply": true,              // Strict bind/call/apply
    "strictPropertyInitialization": true,     // Class init required
    "noImplicitThis": true,                   // Error on 'this' any
    "alwaysStrict": true,                     // Parse strict mode
    "useUnknownInCatchVariables": true        // Catch is 'unknown'
  }
}

Essential Compiler Options

{
  "compilerOptions": {
    // Type Checking
    "exactOptionalPropertyTypes": true,       // Distinguish undefined from missing
    "noFallthroughCasesInSwitch": true,      // Prevent fallthrough in switch
    "noImplicitOverride": true,               // Require 'override' keyword
    "noImplicitReturns": true,                // All code paths must return
    "noPropertyAccessFromIndexSignature": true, // Require bracket notation for index
    "noUncheckedIndexedAccess": true,         // Index signatures return T | undefined
    "noUnusedLocals": true,                   // Error on unused local variables
    "noUnusedParameters": true,               // Error on unused parameters

    // Module Resolution
    "moduleResolution": "bundler",            // Modern bundler resolution
    "resolveJsonModule": true,                // Import JSON files
    "allowImportingTsExtensions": true,       // Import .ts/.tsx files
    "allowSyntheticDefaultImports": true,     // Allow default imports from modules
    "esModuleInterop": true,                  // Emit helpers for CommonJS interop

    // Emit
    "declaration": true,                      // Generate .d.ts files
    "declarationMap": true,                   // Source maps for .d.ts
    "sourceMap": true,                        // Generate .map files
    "removeComments": false,                  // Preserve comments in output
    "importHelpers": true,                    // Import helpers from tslib

    // Interop Constraints
    "isolatedModules": true,                  // Each file can be transpiled separately
    "allowArbitraryExtensions": true,         // Allow imports with any extension
    "verbatimModuleSyntax": false,            // Preserve import/export syntax

    // Skip Checks
    "skipLibCheck": true                      // Skip .d.ts file checking
  }
}