TypeScript里的interface扩展，多继承以及对应的JavaScript代码

interface Shape {
    color: string;
}

interface Square extends Shape {
    sideLength: number;
}

let square = <Square>{};
square.color = "blue";
square.sideLength = 10;

var square = {};
square.color = "blue";
square.sideLength = 10;

interface Shape {
    color: string;
}

interface PenStroke {
    penWidth: number;
}

interface Square extends Shape, PenStroke {
    sideLength: number;
}

let square = <Square>{};
square.color = "blue";
square.sideLength = 10;
square.penWidth = 5.0;

var square = {};
square.color = "blue";
square.sideLength = 10;
square.penWidth = 5.0;