这两个功能之间的区别

Question

While reading "Javascript the Good Parts", came across this bit of code in the "module" chapter.

var serial_maker = function (  ) {

            // Produce an object that produces unique strings. A
            // unique string is made up of two parts: a prefix
            // and a sequence number. The object comes with
            // methods for setting the prefix and sequence
            // number, and a gensym method that produces unique
            // strings.

                var prefix = '';
                var seq = 0;

                return {
                    set_prefix: function (p) {
                        prefix = String(p);
                    },
                    set_seq: function (s) {
                        seq = s;
                    },
                    gensym: function ( ) {
                        var result = prefix + seq;
                        console.log(result);

                        seq += 1;
                        return result;
                    }
                };
            };
var seqer = serial_maker( );
seqer.set_prefix('Q');
seqer.set_seq(1000);
var unique = seqer.gensym( ); // unique is "Q1000

My question is: whats the difference between the above and this bit here:

var other_serial_maker = function(pre, num){

        return pre + num;

    };

    var makeSerial = other_serial_maker("Q", 1000);

Answer 1

If you only objective to to generate the string Q1000 then no difference, but that's not the point. The example from the book is using a closure so that the prefix and seq parts are private and only accessible from within the function.

So the idea is, you can do this:

 var unique = seqer.gensym( ); // unique is "Q1000"

And then you can do this

 var anotherUnique = seqer.gensym( ); // unique is "Q1001"

Because the serial_maker keeps track of it's own state, which your code does not. If we use the code from the book, then after setting up the serial_maker we could call .gensym as many times as we wanted and get a different result. With your code, you'd need to somehow keep track of which codes you've used already.

Answer 2

The main difference is that the outer function scope contains a declaration of prefix and seq , so they are contained in a closure that will follow the seqer object around.

In other words, the example from the book returns an object with a state whereas your example is a plain function (that doesn't use any state).

Answer 3

The point of serial_maker is that it stores state in seq for each serial maker that ensures any one won't produce duplicates. Provided the prefixes are distinct, there won't be duplicates across the serial makers either.