Filter Array of Arrays To Retain Only Elements With Latest Dates Per ID

Question

How to filter this array

    var arr = [[1, "3/20/2018", "A"], [1, "3/20/2018", "B"], 
               [1, "4/20/2018", "A"], [1, "4/20/2018", "B"], 
               [2, "1/30/2018", "A"], [2, "1/30/2018", "B"], 
               [2, "3/30/2018", "A"], [2, "3/30/2018", "B"]
              ];

I want to filter only the elements with the latest dates per other sub-elements (ID) . To have this output:

var result = [[1, "4/20/2018", "A"], [1, "4/20/2018", "B"],
              [2, "3/30/2018", "A"], [2, "3/30/2018", "B"]
             ];

Answer 1

Some observations/assumptions

1. There were missing quotes around the date 3/20/2018. Without quotes this will only be a floating number
2. Your sub-array has 3 items, where the first and last items form a unique combination (ID).

Based on the above, you can try following

var arr = [[1, "3/20/2018", "A"], [1, "3/20/2018", "B"], [1, "4/20/2018", "A"], [1, "4/20/2018", "B"], [2, "1/30/2018", "A"], [2, "1/30/2018", "B"], [2, "3/30/2018", "A"], [2, "3/30/2018", "B"]];
             
   var obj = {};
   
   arr.forEach(function(item){
     var id = item[0] + "_" + item[2];
     if(obj[id]) {
       if(new Date(item[1]) > new Date(obj[id][1])) {
          obj[id] = item;  
       }
     } else {
       obj[id] = item;
     }
  });
   
  console.log(Object.values(obj));