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;
};

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);
  }
};