object comparison with recursion

gistfile1.js

var compare = function(a, b) {
  "use strict";
  // iterate over each key in b
  for (var key in b) {
    // check if the key is present in a
    if (key in a) {
      // make sure a and be are not exact matches
      if (b[key] !== a[key]) {
        if (typeof b[key] === "object") {
          // if b[key] is an object recursively call compare
          compare(a[key], b[key]);
        } else {
          // if not an exact match add to c
          c[key] = b[key];
        }
      }
    } else {
      // if the key is not found add it to c
      c[key] = b[key];
    }
  }
  return c;
};

Consider looking into Javascript instanceof operator and toString function. That may help you determine the type of object that you're working with.

Also, what happens when you run this the first time? Is there anywhere that it would error out?

Thanks @polastre!

Regarding the error, it would error if either a or b are not an object, so it would need to check that up front.

Something along the lines of:

var objCheck = function(a, b) {
  if ( a === null ||
       typeof a !== "object" ||
       b === null ||
       typeof b !== "object"
     ) {
    throw "Must compare two objects";
  } else {
    compare(a,b);
  }
};