typescript-inference

# TypeScript Inference Skill

Patterns for leveraging TypeScript's type inference capabilities.

## Basic Inference

### Variable Inference

```typescript
// TypeScript infers types from initialization
const name = 'John'          // type: string
const age = 30               // type: number
const isActive = true        // type: boolean
const items = [1, 2, 3]      // type: number[]

// Literal types with const
let status = 'active'        // type: string
const mode = 'dark' as const // type: 'dark'

// Object inference
const user = {
  name: 'John',
  age: 30,
}  // type: { name: string; age: number }

// const assertion for readonly literal object
const config = {
  api: '/api/v1',
  timeout: 5000,
} as const
// type: { readonly api: "/api/v1"; readonly timeout: 5000 }
```

### Function Return Inference

```typescript
// Return type inferred from return statements
function add(a: number, b: number) {
  return a + b  // return type: number
}

function getUser(id: string) {
  return { id, name: 'John', active: true }
}  // return type: { id: string; name: string; active: boolean }

// Async function inference
async function fetchData() {
  const response = await fetch('/api/data')
  return response.json() as Promise<Data>
}  // return type: Promise<Data>
```

### Generic Inference

```typescript
// Type parameter inferred from argument
function identity<T>(value: T): T {
  return value
}

const str = identity('hello')  // T inferred as string
const num = identity(42)       // T inferred as number

// Multiple type parameters
function pair<T, U>(first: T, second: U) {
  return [first, second] as const
}

const result = pair('name', 42)  // [T, U] inferred as [string, number]
```

## Contextual Typing

### Callback Parameters

```typescript
// Parameter types inferred from context
const numbers = [1, 2, 3, 4, 5]

// 'num' inferred as number from array type
const doubled = numbers.map(num => num * 2)

// Event handler inference
document.addEventListener('click', event =