Enforcing new 模式

// 构造函数

function Waffle () {

this . tastes = "yummy" ;

}

// 反例

// 忘记使用 `new`

var good_morning = Waffle ();

console . log ( typeof good_morning ); // "undefined"

console . log ( window . tastes ); // "yummy" 此时 构造函数Waffle中的this指代window

//更改为使用new关键字

var good_morning = new Waffle ();

console . log ( typeof good_morning ); // "object"

console . log ( good_morning . tastes ); // "yummy"

// 强制使用new

function Waffle () {

if ( ! ( this instanceof Waffle )) {

return new Waffle ();

}

this . tastes = "yummy" ;

}

var good_morning = new Waffle ();

var good_evening = Waffle ();

console . log ( typeof good_morning ); // "object"

console . log ( good_morning . tastes ); // "yummy"

console . log ( typeof good_evening ); // "object"

console . log ( good_evening . tastes ); // "yummy"