The == operator compares objects by identity. But sometimes, you would prefer to compare the values of their actual properties. Write a function, deepEqual, that takes two values and returns true only if they are the same value or are objects with the same properties whose values are also equal when compared with a recursive call to deepEqual. T…

4-4-deepcomparison.js

/*
http://eloquentjavascript.net/04_data.html

The == operator compares objects by identity. But sometimes, you would prefer to compare the values of their actual properties.

Write a function, deepEqual, that takes two values and returns true only if they are the same value or are objects with the same properties whose values are also equal when compared with a recursive call to deepEqual.

To find out whether to compare two things by identity (use the === operator for that) or by looking at their properties, you can use the typeof operator. If it produces "object" for both values, you should do a deep comparison. But you have to take one silly exception into account: by a historical accident, typeof null also produces "object".
*/

function deepEqual( obj1, obj2 ) {
  // Check if both values are equivalent
  if( obj1 === obj2 ) return true;
  
  if( obj1 == null || typeof obj1 != "object" || obj2 == null || typeof obj2 != "object" ) return false;

  var propsObj1 = 0, propsObj2 = 0;

  for( var prop in obj1 )
    propsObj1++;

  for( var prop in obj2 ) {
    propsObj2++;

    if( !(prop in obj1) || !deepEqual( obj1[prop], obj2[prop] ) )
      return false;
  }
  
  return propsObj1 === propsObj2;
}

var obj = {here: {is: "an"}, object: 2};
console.log(deepEqual(obj, obj));
// → true
console.log(deepEqual(obj, {here: 1, object: 2}));
// → false
console.log(deepEqual(obj, {here: {is: "an"}, object: 2}));
// → true