What is process.memory giving ?

To me is now becoming more clear how the Javascript interpreter works looking at how this simple example code has been parsed and put into memory structures.
I am now asking myself which should be the best way to use Espruino.
In example adding a simple toFixed function would require adding a prototype to Number Class, which is too complex with respect using an existing native C sprintf function, even a big piece of C code itself in the real world.
Probably the best use of Espruino could be to act as a scripted shell coprocessor of a traditional CPU, doing more intensive computations ( i.e. another STm32Fx micro controller ).
It seems that there are pro and cons using Espruino.
Is there a place where to find good books about the theory of a Javascript interpreter or handy manuals about how to build one.
Looking inside the Espruino code should give enough explanation, but to a novice this is the hardest way.

var Circle = function(radius) {
  this.radius = radius;
};

Circle.prototype.area = function() {
  return Math.PI * this.radius* this.radius;
};

var a = new Circle(3);
var b = new Circle(4);
var c = new Circle(1);

a.area(); // .toFixed(2);
b.area(); // .toFixed(2);
c.area();

#1[r1,l1] Object {
  #4[r1,l0] Name: '>timers'  #5[r2,l0] Array [
  ]
  #6[r1,l0] Name: '>watches'  #7[r2,l0] Array [
  ]
  #9[r1,l0] Name: '>history'  #10[r1,l0] Array [
    #11[r1,l0] Name: int 0  #8[r1,l0] String echo(0);
    #24[r1,l0] Name: int 1  #16[r1,l0] String var Circle = function(radius) {\n  this.radius = radius;\n};
    #41[r1,l0] Name: int 2  #30[r1,l0] String Circle.prototype.area = function() {\n  return Math.PI * this.radius* this.radius;\n};
    #50[r1,l0] Name: int 3  #43[r1,l0] String var a = new Circle(3);
    #61[r1,l0] Name: int 4  #54[r1,l0] String var b = new Circle(4);
    #72[r1,l0] Name: int 5  #65[r1,l0] String var c = new Circle(1);
    #75[r1,l0] Name: int 6  #77[r1,l0] String a.area();  
   #82[r1,l0] Name: int 7  #83[r1,l0] String b.area();  
    #86[r1,l0] Name: int 8  #76[r1,l0] String c.area();
    #88[r1,l0] Name: int 9  #89[r1,l0] String echo(1);
  ]

  #12[r4,l0] Name: 'Circle'  #13[r1,l0] Function {
    #14[r1,l0] Param Name: 'radius'  undefined
    #23[r1,l0] Name: '>code'  
    #15[r1,l0] String {\n  this.radius = radius;\n}
    #26[r4,l0] Name: 'prototype'  #25[r1,l0] Object {
      #28[r1,l0] Name: 'area' 
      #29[r1,l0] Function {
        #40[r1,l0] Name: '>code'  
        #36[r1,l0] String {\n  return Math.PI * this.radius* this.radius;\n}
      }
    }
  }
  #42[r1,l0] Name: 'a'  #45[r1,l0] Object {
    #46[r1,l0] Name: '__proto__'  #26[r4,l0] ...
    #52[r1,l0] Name: 'radius'     #49[r1,l0] Integer 3
    #48[r1,l0] Name: 'constructor'  #12[r4,l0] ...
  }
  #53[r1,l0] Name: 'b'  #56[r1,l0] Object {
    #57[r1,l0] Name: '__proto__'  #26[r4,l0] ...
    #63[r1,l0] Name: 'radius'  #60[r1,l0] Integer 4
    #59[r1,l0] Name: 'constructor'  #12[r4,l0] ...
  }
  #64[r1,l0] Name: 'c'  #67[r1,l0] Object {
    #68[r1,l0] Name: '__proto__'  #26[r4,l0] ...
    #74[r1,l0] Name: 'radius'  #71[r1,l0] Integer 1
    #70[r1,l0] Name: 'constructor'  #12[r4,l0] ...
  }
  #81[r1,l0] Name: 'Math'  #80[r1,l0] Function {
  }
}

process.memory()

=
{"free" :5394,
 "usage":56,
 "total":5450,
 "history":40,
 "stackEndAddress" :536983744,
 "flash_start"     :134217728,
 "flash_binary_end":134404416,
 "flash_code_start":135135232,
 "flash_length"    :  1048576}