user defined object equality for a set in harmony (es6)

I have a problem where I'm generating many values and need to make sure I only work with unique ones. Since I'm using node js, with the --harmony flag, and have access to harmony collections, I decided that a Set may be an option. What I'm looking for is something similar to the following example: ``` 'use strict'; function Piece(x,y){ this.x = x this.y = y } function Board(width,height,pieces){ this.width = width this.height = height this.pieces = pieces } function generatePieces(){ return [ new Piece(0,0), new Piece(1,1) ] } //boardA and boardB are two different but equivalent boards var boardA = new Board(10,10,generatePieces()) var boardB = new Board(10,10,generatePieces()) var boards = new Set() boards.add(boardA) boards.has(boardB) //return true ``` Now normally to achieve this in another language, say c#, I would expect to have to implement an equals function, as well as a hash code generating function for both Board and Piece. Since I'd expect the default object equality to be based on references. Or perhaps use a special immutable value type (say, a case class in scala) Is there a means to define equality for my objects to solve my problem?