# 泛型

## 简单的例子

``````function createArray(length: number, value: any): Array<any> {
let result = [];
for (let i = 0; i < length; i++) {
result[i] = value;
}
return result;
}

createArray(3, 'x'); // ['x', 'x', 'x']
``````

`Array<any>` 允许数组的每一项都为任意类型。但是我们预期的是，数组中每一项都应该是输入的 `value` 的类型。

``````function createArray<T>(length: number, value: T): Array<T> {
let result: T[] = [];
for (let i = 0; i < length; i++) {
result[i] = value;
}
return result;
}

createArray<string>(3, 'x'); // ['x', 'x', 'x']
``````

``````function createArray<T>(length: number, value: T): Array<T> {
let result: T[] = [];
for (let i = 0; i < length; i++) {
result[i] = value;
}
return result;
}

createArray(3, 'x'); // ['x', 'x', 'x']
``````

## 多个类型参数

``````function swap<T, U>(tuple: [T, U]): [U, T] {
return [tuple[1], tuple[0]];
}

swap([7, 'seven']); // ['seven', 7]
``````

## 泛型约束

``````function loggingIdentity<T>(arg: T): T {
console.log(arg.length);
return arg;
}

// index.ts(2,19): error TS2339: Property 'length' does not exist on type 'T'.
``````

``````interface Lengthwise {
length: number;
}

function loggingIdentity<T extends Lengthwise>(arg: T): T {
console.log(arg.length);
return arg;
}
``````

``````interface Lengthwise {
length: number;
}

function loggingIdentity<T extends Lengthwise>(arg: T): T {
console.log(arg.length);
return arg;
}

loggingIdentity(7);

// index.ts(10,17): error TS2345: Argument of type '7' is not assignable to parameter of type 'Lengthwise'.
``````

``````function copyFields<T extends U, U>(target: T, source: U): T {
for (let id in source) {
target[id] = (<T>source)[id];
}
return target;
}

let x = { a: 1, b: 2, c: 3, d: 4 };

copyFields(x, { b: 10, d: 20 });
``````

## 泛型接口

``````interface SearchFunc {
(source: string, subString: string): boolean;
}

let mySearch: SearchFunc;
mySearch = function(source: string, subString: string) {
return source.search(subString) !== -1;
}
``````

``````interface CreateArrayFunc {
<T>(length: number, value: T): Array<T>;
}

let createArray: CreateArrayFunc;
createArray = function<T>(length: number, value: T): Array<T> {
let result: T[] = [];
for (let i = 0; i < length; i++) {
result[i] = value;
}
return result;
}

createArray(3, 'x'); // ['x', 'x', 'x']
``````

``````interface CreateArrayFunc<T> {
(length: number, value: T): Array<T>;
}

let createArray: CreateArrayFunc<any>;
createArray = function<T>(length: number, value: T): Array<T> {
let result: T[] = [];
for (let i = 0; i < length; i++) {
result[i] = value;
}
return result;
}

createArray(3, 'x'); // ['x', 'x', 'x']
``````

## 泛型类

``````class GenericNumber<T> {
zeroValue: T;
add: (x: T, y: T) => T;
}

let myGenericNumber = new GenericNumber<number>();
myGenericNumber.zeroValue = 0;
myGenericNumber.add = function(x, y) { return x + y; };
``````

## 泛型参数的默认类型

``````function createArray<T = string>(length: number, value: T): Array<T> {
let result: T[] = [];
for (let i = 0; i < length; i++) {
result[i] = value;
}
return result;
}
``````