javascript 'curry', need some explanations on this code

Question

I am reading the book "JavaScript-The Good Parts", in chapter 4.14 Curry, the book gives the following example:

Function.method('curry', function(){
    var slice = Array.prototype.slice,
        args = slice.apply(arguments), //1st-arguments
        that=this;

    return function(){
       return that.apply(null, args.concat(slice.apply(arguments))); //2nd-arguments
    }
})

var add1=add.curry(1);
document.writeln(add1(6)); // 7

I have two questions on this code:

1. There are two places using 'arguments'. In the method invoking in the last two lines code, is it so that '1' goes to the 1st-arguments and '6' goes to the 2nd-arguments?

2. There is a line of code apply(null, args.concat(slice.apply(arguments))), why does it apply(null,...) here?, what is the sense to apply a argument to a null object?

Answer 1

There are two places using 'arguments'. In the method invoking in the last two lines code, is it so that '1' goes to the 1st-arguments and '6' goes to the 2nd-arguments?

Yes, 1 is part of arguments of the outer function, while 6 goes the arguments in the inner function. The inner function can capture all other variables in a closure except for arguments and this, which have a special meaning inside a function, hence are not part of that closure. So that, and args are captured, but not arguments.

There is a line of code apply(null, args.concat(slice.apply(arguments))), why does it apply(null,...) here?, what is the sense to apply a argument to a null object?

Invoking a method with a null context will simply set the this value to the global object. In the case it seems the author does not care what that context is.