Spy function

gistfile1.js

var pSlice = Array.prototype.slice;

function createSpy(fn) {
    
    function spy() {
        // clone and coerce arguments into an array
        var spiedArgs = pSlice.call(arguments);
        // remember the arguments the spy was called with
        spy.calls.push(spiedArgs);
        // call the "behaviour" function passed when creating this
        // spy with the actual arguments passed at runtime.
        return fn && fn(spiedArgs);
    }
    
    // memo of calls made to the spy
    spy.calls = [];

    return spy;

}