Comparing Two Javascript Objects

Recently, I was faced with a problem where I needed to compare two Javascript objects. My initial strategy was to convert them to JSON and compare the JSON strings.

Sort of like this:

var a = JSON.stringify(person1);//'{"firstName":"JP","lastName":"Richardson"}'
var b = JSON.stringify(person2);//'{"firstName":"JP","lastName":"Richardson"}'

assert(a === b);

Simple enough, right?

Not so fast. I encountered a case like this:

var a = JSON.stringify(person1);//'{"firstName":"JP","lastName":"Richardson"}'
var b = JSON.stringify(person2);//'{"lastName":"Richardson","firstName":"JP"}'

assert(a === b);

The data is the same, but the string is different. Fortunately, Stackoverflow had a nice Javascript object comparison algorithm to dump into my app.

Object.prototype.equals = function(x)
  var p;
  for(p in this) {
      if(typeof(x[p])=='undefined') {return false;}

  for(p in this) {
      if (this[p]) {
          switch(typeof(this[p])) {
              case 'object':
                  if (!this[p].equals(x[p])) { return false; } break;
              case 'function':
                  if (typeof(x[p])=='undefined' ||
                      (p != 'equals' && this[p].toString() != x[p].toString()))
                      return false;
                  if (this[p] != x[p]) { return false; }
      } else {
          if (x[p])
              return false;

  for(p in x) {
      if(typeof(this[p])=='undefined') {return false;}

  return true;

Test passed. I eventually hit a situation where I had some code with an Object that had a Person prototype and some data that came from JSON. Kinda like this:

var person1 = new Person('JP', 'Richardson');
var person2 = JSON.parse('{"firstName":"JP","lastName":"Richardson"}');

//deepEquals is code snippet above ^
person1.deepEquals(person2); // <--- THIS FAILS

I only cared about comparing the data. The methods associated with the object (Prototype) didn't matter. Let's modify the above algorithm. I use CoffeeScript. Here's the modification:

Object::jsonEquals = (x) ->
  #we do this because two objects may have the same data fields and data but different prototypes
  x1 = JSON.parse(JSON.stringify(this))
  x2 = JSON.parse(JSON.stringify(x))

  p = null
  for p of x1
    return false if typeof (x2[p]) is 'undefined'
  for p of x1
    if x1[p]
      switch typeof (x1[p])
        when 'object'
          return false unless x1[p].jsonEquals(x2[p])
        when 'function'
          return false if typeof (x2[p]) is 'undefined' or (p isnt 'equals' and x1[p].toString() isnt x2[p].toString())
          return false  unless x1[p] is x2[p]
      return false if x2[p]
  for p of x2
    return false if typeof (x1[p]) is 'undefined'

This causes the situation like I described above to pass. Essentially convert to JSON to remove the prototype. I suppose you could make this more efficient my just manually setting the prototype to Object before doing the comparison, but oh well this works for the time being.

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