simple object diff: less than 300 bytes compressed

json_diff

(function diff(a,b) {
  var c = JSON.stringify,
    i = function (x) {return x==null||typeof x!='object'};
  return (function d(a, b, p) {
    var s, k, t='';
    if (i(a)||i(b))
      return a !==b ? p+'-'+c(b)+p+'+'+c(a) : '';
    for (k in a)
      t += (k in b)
        ? ((s = d(a[k], b[k], p+'  ')) ? p+k+':'+s : '')
        : p+'+'+k+':'+c(a[k])
    for (k in b)
      t += (k in a) ? '' : p+'-'+k+':'+c(b[k]);
    return t
  }(a,b,'\n'))
}({a:1,b:2,c:3,d:4},{a:1,b:{c:3},c:3}))
/*"
b:
  -{"c":3}
  +2
+d:4"*/