typeScript 单例模式

在创建一个数据库实例的时候，我们不需要每个请求都创建一个实例，所以在设计类的时候，使用单例模式，同时使用TypeScript的静态方法，和静态属性

interface DBOperations {
    query(str:string):boolean
    insert(str:string):boolean
    delete(str:string):boolean
    update(str:string):boolean
}

class MySQL implements DBOperations{
    private static instance:MySQL
    host:string
    port:number
    userName:string
    password:string
    dbName:string
    private constructor(host:string='127.0.0.1',port:number = 3306, userName:string='root',password:string,dbName:string='homeStread'){
        this.host = host
        this.port = port
        this.userName = userName
        this.password = password
        this.dbName = dbName
    }
    toString(){
        let _str =  `
        host:${this.host},
        port:${this.port},
        dbName:${this.dbName},
        userName:${this.userName},
        password:${this.password}
        `
        console.log(_str)
        return _str
    }
    static  getInstance() {
        if (!this.instance) {
            // @ts-ignore
            this.instance = new MySQL()
        }
        return this.instance
    }

    delete(str: string): boolean {
        console.log(`query:${str}`)
        return false;
    }

    insert(str: string): boolean {
        console.log(`delete:${str}`)
        return false;
    }

    query(str: string): boolean {
        console.log(`query:${str}`)
        return false;
    }

    update(str: string): boolean {
        console.log(`update:${str}`)
        return false;
    }
}

let mySQLInstance = MySQL.getInstance()
mySQLInstance.password = 'secret'
mySQLInstance.toString()
mySQLInstance.query('select * from table')