What's the purpose of returning a function from a function?

Question

Reading some legacy code, and found

A.prototype.setSize: function () {
   var v1 = new Vector2();
   return function (size ) {

     var halfSize = v1.copy( size ).multiplyScalar( 0.5 );
     this.min.sub( halfSize );
     return this;

   };

}(),

I am wondering:

1. why define setSize as a function which return another function
2. Also the defined function is executed right away.

any light to shed on this?

Updated:

I can simply use

   A.prototype.setSize: function (size) {
       var v1 = new Vector2();

       var halfSize = v1.copy( size ).multiplyScalar( 0.5 );
       this.min.sub( halfSize );
       return this;

    },

Is the first snippet better than second?

Answer 1

1. So the returned function can access the value of v1 each time it is called without making v1 a global (or otherwise non-private)
2. That is so the returned function is assigned to setSize

Answer 2

It function closure. Used to hide variables from global scope. Read on and see some examples here

JS function closures

Answer 3

The purpose in that particular case is to avoid creating a new Vector object each time setSize is called. It's a caching strategy. The alternative would be to just have a flat function:

A.prototype.setSize: function (size) {
  var v1 = new Vector2();
  var halfSize = v1.copy(size).multiplyScalar(0.5);
  this.min.sub(halfSize);
  return this;
}

I would only use the closure cached version of setSize if I found I was having performance problems because of new Vector2().