SOURCE

// 不改变原有对象的属性和方法，为它添加新的属性和方法
var b = {
    bb: function() {
        console.log('bb')
    }
}

var decorator = function(obj,fn) {
    if(typeof obj.aa === 'function') {
        const oldFn = obj.aa
        obj.aa = function() {
            oldFn()
            fn()
        }
    } else {
        obj.aa = fn
    }
}

decorator(b,function() {
    console.log('aa')
})

b.aa()

console.log(b)