typescript范型

范型：支持不特定的类型，要求传入的参数和返回的参数保持一致。
1 基本例子

function identity<T>(arg: T): T {
    return arg;
}
let output = identity<string>("myString"); 
let output = identity("myString"); 

2 泛型变量

function loggingIdentity<T>(arg: T[]): T[] {
    console.log(arg.length);  // Array has a .length, so no more error
    return arg;
}
// 也可以这样
function loggingIdentity<T>(arg: Array<T>): Array<T> {
    console.log(arg.length);  // Array has a .length, so no more error
    return arg;
}
loggingIdentity(['2'])

3 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; };

4 泛型约束

interface Lengthwise {
    length: number;
}
function loggingIdentity<T extends Lengthwise>(arg: T): T {
    console.log(arg.length);  // Now we know it has a .length property, so no more error
    return arg;
}
loggingIdentity(3);  // Error, number doesn't have a .length property

5 范型接口

interface Fn<T>{
    (value:T):T;
}
function print<T>(value:T):T{

    return value;
}
const printData:Fn<string>=print;