# 型別推論

如果沒有明確的指定型別，那麼 TypeScript 會依照型別推論（Type Inference）的規則推斷出一個型別。

## 什麼是型別推論

以下程式碼雖然沒有指定型別，但是會在編譯的時候報錯：

```typescript
let myFavoriteNumber = 'seven';
myFavoriteNumber = 7;

// index.ts(2,1): error TS2322: Type 'number' is not assignable to type 'string'.
```

事實上，它等價於：

```typescript
let myFavoriteNumber: string = 'seven';
myFavoriteNumber = 7;

// index.ts(2,1): error TS2322: Type 'number' is not assignable to type 'string'.
```

TypeScript 會在沒有明確的指定型別的時候推測出一個型別，這就是型別推論。

**如果定義的時候沒有賦值，不管之後有沒有賦值，都會被推斷成 `any` 型別而完全不被型別檢查**：

```typescript
let myFavoriteNumber;
myFavoriteNumber = 'seven';
myFavoriteNumber = 7;
```

## 參考

* [Type Inference](http://www.typescriptlang.org/docs/handbook/type-inference.html)（[中文版](https://zhongsp.gitbooks.io/typescript-handbook/content/doc/handbook/Type%20Inference.html)）
* [上一章：任意值](/typescript-tutorial/basics/any.md)
* [下一章：聯合型別](/typescript-tutorial/basics/union-types.md)


---

# 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://willh.gitbook.io/typescript-tutorial/basics/type-inference.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.