# 元组

数组合并了相同类型的对象，而元组（Tuple）合并了不同类型的对象。

元组起源于函数编程语言（如 F#），这些语言中会频繁使用元组。

## 简单的例子

定义一对值分别为 `string` 和 `number` 的元组：

```typescript
let tom: [string, number] = ['Tom', 25];
```

当赋值或访问一个已知索引的元素时，会得到正确的类型：

```typescript
let tom: [string, number];
tom[0] = 'Tom';
tom[1] = 25;

tom[0].slice(1);
tom[1].toFixed(2);
```

也可以只赋值其中一项：

```typescript
let tom: [string, number];
tom[0] = 'Tom';
```

但是当直接对元组类型的变量进行初始化或者赋值的时候，需要提供所有元组类型中指定的项。

```typescript
let tom: [string, number];
tom = ['Tom', 25];
```

```typescript
let tom: [string, number];
tom = ['Tom'];

// Property '1' is missing in type '[string]' but required in type '[string, number]'.
```

## 越界的元素

当添加越界的元素时，它的类型会被限制为元组中每个类型的联合类型：

```typescript
let tom: [string, number];
tom = ['Tom', 25];
tom.push('male');
tom.push(true);

// Argument of type 'true' is not assignable to parameter of type 'string | number'.
```

## 参考

* [Basic Types # Tuple](http://www.typescriptlang.org/docs/handbook/basic-types.html#tuple)（[中文版](https://zhongsp.gitbooks.io/typescript-handbook/content/doc/handbook/Basic%20Types.html#元组-tuple)）


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