javascript - 在函数上调用 apply()

Question

我有以下代码，那么当调用squareMem(10)时，f.apply(this,arguments)中的this是做什么的指的是？

function square(num){
    return num*num;
}

function memoize(f){
                var cache = {};

                return function(){

                    var key = JSON.stringify(Array.prototype.slice.call(arguments));

                    if(key in cache){
                        console.log('From cache...');
                        return cache[key];
                    }else{
                        console.log('Computing..');
                        return cache[key] = f.apply(this,arguments); // what does `this` refer to?
                    }

                }

            }

var squareMem = memoize(square);
squareMem(10);//100

Answer 1

this 将引用调用内存函数的上下文。仅当原始函数使用 this 时，它才真正有用:

function square2(num) {
   return this + ": " + (num*num);
}

var o = { };
var d = new Date();
var f = memoize(square2);
o.f = memoize(square2);
d.f = memoize(square2);

f(20)   => "[object Window]: 400"
o.f(20) => "[object Object]: 400"
d.f(20) => "Sun Apr 27 2014 15:03:41 GMT-0500 (CDT): 400"