Question

I am working my way through Eloquent Javascript and I came across a code snippet that looks like this:

function greaterThan(x) {
  return function(y) {
    return y > x;
  };
}

var greaterThanTen = greaterThan(10);
show(greaterThanTen(9));

Is there any real use-case to defining the function like this with an anonymous function inside of it? Wouldn’t it be much simpler to do this:

function greaterThan(x,y){
  return x > y;
}

var greaterThanTen = greaterThan(9, 10);

any ideas/comments/suggestions would be very helpful.

Answer 1

This is an example of a “closure”. Basically, a call to greaterThan() gives you a function. That function, instead of just a mere function, carries with it the value of x – it’s like embedding the value of x permanently into the function.

function obj(x) {

    //"public interface"
    //only these are exposed (inc and getX). x is private.
    return {
        inc: function() {
            return ++x;
        },
        getX: function() { //a getter function to retrieve private x
            return x;
        }
    }
}

//the value 10 is "embedded" to the returned object
var myobj = obj(10);

//every call to inc increments that "embedded" value
console.log(myobj.inc()); //11
console.log(myobj.inc()); //12
console.log(myobj.inc()); //13

//but you can never access the variable of `x` directly without a "getter"
console.log(myobj.getX());​

Closures are one of the great features of JavaScript. One of it’s great uses is to emulate private variables.