snippet.js

function buffered_write(func) {
  var old_write = document.write;
  var buffer = '';
  document.write = function(string) {
    buffer += string;
  };
  func();
  document.write = old_write;
  return buffer;  
}

document.write('Original, before');

var result = buffered_write(function() {
  document.write('I am overloaded');
  document.write('Here I am again');
});

console.log('Result:');
console.log(result);

document.write('Original, after');