Generics and generic constraints of typescript

One 、 Generic

In the use of ts When encapsulating a function , Because the input parameter is not fixed , The inevitable parameter type cannot be fixed , At this point, you need to use generics to define parameter types . Generics can be understood as broad types , Usually used for classes and functions .

example 1. Class

class Person<T>{
    private name: T;
    constructor(value: T) {
        this.name = value;
    }
}
const p = new Person<string>('Dalon')

example 2. Use generics in functions

function identity<T>(data: T): T {
    console.log(data)
    return data;
}
const id1 = identity<string>('hey boy!');
const id2 = identity<number>(123456);

example 3. Use generics in interfaces

interface Person<T1=string, T2=number> {
    name: T1
    age: T2
}
const dalon:Person = {
    name: 'dalon',
    age: 18
}
const tom:Person<string, string> = {
    name: 'Tom',
    age: '18'
}

  example 3 Medium T1 and T2 Generics add default types , Similar to the default parameters of a function .

Two 、 Generic constraint

  Generic constraint is to control the type of generic , If it is limited to object Type or specify interface type . When using properties or methods of generic parameters in functions , You need to constrain generics .

example 1. Through interface constraints

interface Lengthwise {
    length: number;
}

function loggingIdentity<T extends Lengthwise>(arg: T): T {
    console.log(arg.length);     //  without Lengthwise Generic constraint , Will report a mistake Error: T doesn't have .length
    return arg;
}

example 2. object constraint

function getKeys<T extends object>(data:T) :string[] {
    const keys:Array<string> = Object.keys(data)
    return keys
}

Above example 2 Of getKeys Method to get all attribute names of an object ,  because Object.keys yes object Prototype method of , So ask for data It has to be a object type .

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reference ：

Generic · TypeScript Chinese net · TypeScript——JavaScript Superset