# Type predicate

## readonly

해당 배열, 객체를 수정할 수 없고 오직 읽을 수만 있게 함.

* 배열은 표현 방법이 `string[]`, `Array<string>` 2가지 이지만, `readonly`를 사용할 때는 `string[]` 표현식만 가능하다.

```typescript
function printArray(fruits: readonly string[]) {}
```

## | (Union Type)

a 또는 b 또는 c 또는 d가 올 수 있다는 뜻.

```typescript
type Direction = 'left' | 'right' | 'up' | 'down';
```

## & (Intersection type)

모든 것을 다 합한 의미로 사용 a도 b도 c도 d도 다 있어야 한다는 &#x20;

```typescript
type Student = {
  name: string;
  score: number;
};

type Worker = {
  empolyeeId: number;
  work: () => void;
};

function internWork(person: Student & Worker) {
  console.log(person.name, person.empolyeeId, person.work());
}

```

## in

우항에 있는 타입의 속성을 가지고 있는지 확인

```typescript
type SuccessState = {
  response: {
    body: string;
  };
};
type FailState = {
  reason: string;
};
type LoginState = SuccessState | FailState;


function printLoginState(state: LoginState) {
  if ('response' in state) {
    console.log(`🎉 ${state.response.body}`);
  } else {
    console.log(`😭 ${state.reason}`);
  }
}
```

## as (Type Assertion)

타입을 명시적으로 지정해 줌

```typescript
function jsStrFunc(): any {
    return 2;
}
const result = jsStrFunc();
console.log((result as string).length);
console.log((<string>result).length);
```

## !

절대적으로 값이 있음을 확신할 때 표현

```typescript
function findNumbers(): number[] | undefined {
    return undefined;
  }
  const numbers = findNumbers()!;
  // or
  numbers!.push(2);
```

## is

A가 B타입인지 아닌지 확인

```typescript
A is B // true or false
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://dev-sujeong.gitbook.io/typescript/type-predicate.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.