一个Class的属性是另一个Class的类型, 如何初始化?

Creator 2.x
#1

AAA.js

var BBB = require("BBB");
cc.Class({
    extends: cc.Component,
    properties: {
        bbb: {
            default: null,
            type: BBB,
        }
    },
    onLoad: function () {
        bbb = new BBB();
    },    
});

BBB.js

var BBB = cc.Class({
    name: "BBB",
...
});
module.exports = BBB;

为什么运行报错 Uncaught ReferenceError: bbb is not defined

#2

AAA.js的onLoad中：
this.bbb = new BBB();