对间谍对象的非法调用

Question

I am trying to create a Spy object which will monitor how many times a method on another object gets called. I can get the monitoring working by wrapping the target's method in my Spy , but when I try to invoke the target's method, I get an Illegal invocation error, and I'm unsure why.

function Spy(target, method) {
    var counter = 0;
    var oldFunc = target[method];
    target[method] = function(args){
        return (function(){
            counter++;
            oldFunc(args); //ILLEGAL
        })();
    }
    return { count : counter }
}
var spy = Spy(console, 'error');
console.error('foo', 'bar');
console.error('foobar');
console.log(spy.count);

http://jsfiddle.net/s505eemb/

Answer 1

Main problems with your function invocation are:

• you're not passing all arguments
• you're not passing the expected this value
• you're not returning the result value

Use the apply method to fix this. Btw, that IEFE is useless, and you're not updating the .count property, but only the local counter variable.

function Spy(target, method) {
    var counter = 0;
    var oldFunc = target[method];
    target[method] = function() {
        counter++;
        return oldFunc.apply(this, arguments);
    };
    return { get count() { return counter; } }
}

( updated demo )