Finding identical keys in multiple objects

function flipObj( i ) {
  
  // If the input variable is not an object, fail.
  if( typeof i !== "object" ) return false;

  // If the input object is empty, fail
  if( Object.keys( i ).length === 0 ) return false;
  
  // Create an empty object.
  o = {};

  // Loop through the dates
  for( var d in i ) {
    
    // If the property in question is not an object, skip it.
    if( typeof i[d] !== "object" ) continue;
    
    // If the property is empty, skip it.
    if( Object.keys( i[d] ).length === 0 ) continue;

    // Loop through the email addresses
    for( var e in i[d] ) {

      // If there is no property in our created object for the email, make one
      if(!o[e]) o[e] = { days: [], id: i[d][e] };
      
      // Push the day into the days array for the email object in question.
      o[e].days.push( d );

    }
  }

  // If our new object is empty, fail
  if( Object.keys( o ).length === 0 ) return false;

  // Return our new object.
  return o;
}

// Works
console.log( flipObj( { '2015-08-09': { '[email protected]': 5, '[email protected]': 4, '[email protected]': 6 }, '2015-09-09': { '[email protected]': 5, '[email protected]': 4, '[email protected]': 6, '[email protected]': 4 }, '2015-10-09': { '[email protected]': 5, '[email protected]': 4, '[email protected]': 6, '[email protected]': 10, '[email protected]': 7 } } ) ); 
// Fails
console.log( flipObj( {} ) ); 
// Fails
console.log( flipObj( { 'hello': 'world' } ) );