How to convert multidimensional array to this format

Question

I have data coming in every second from a Web Socket eg

1- [["X",1],["Y",2],["Z",3]] 

2 -[["X",2],["Y",7]] 

3 -[["Y",5],["Z",1]] 

4 -[["X",7]] 
...

The resultant array for each iteration

1 - ["X",1,0],["Y",2,0],["Z",3,0]] // 0 is nothing but the difference it can also be + or minus

2 - ["X",2,1],["Y",7,5],["Z",3,0]] // diff from first iteration

3 - ["X",1,0],["Y",5,-2],["Z",1,-2]] // diff from second

the things i have tried till now

    this.socketSubscription = this.socket.messages.subscribe((message) => {
      this.prev = this.rows;

      this.rows = JSON.parse(message);
      if(this.prev){
        this.rows.forEach(element => {
          for (var index = 0; index < element.length; index++) {
            console.log(element[index]);
            let check = this.prev.find(prevElement => prevElement.find(el => el[0]));
            console.log("check"+check);
/*             if (element[0] === ())){
              console.log("here");
            } */
          }
        });
      }

Do you have to compare values on index or on keys? You are getting different keys each time as I can see — Surender Khairwa
– Surender Khairwa, Commented Oct 4, 2017 at 19:30
i need to compare the incoming values with the prev and if there is a cahnage jsut list that and keep on doing it — INFOSYS
– INFOSYS, Commented Oct 4, 2017 at 19:34

aaron · Accepted Answer · 2017-10-05 09:30:07Z

You don't need the for loop or a nested find. This is close enough and should get you on track:

var results;

function process(data) {
    if (results) {
        data.forEach(element => {
            var key = element[0];
            var val = element[1];
            var index = results.findIndex(result => result[0] == key);
            var prevVal = results[index][1];
            var diff = val - prevVal;
            results[index][1] = val;
            results[index][2] = diff;
        });
    } else {
        results = data.map(element => { element[2] = 0; return element; });
    }
}

var a = [["X",1],["Y",2],["Z",3]];
var b = [["X",2],["Y",7]];
var c = [["Y",5],["Z",1]];

process(a); console.log(results); // [["X",1,0], ["Y",2, 0], ["Z",3, 0]]
process(b); console.log(results); // [["X",2,1], ["Y",7, 5], ["Z",3, 0]]
process(c); console.log(results); // [["X",2,0], ["Y",5,-2], ["Z",1,-2]]

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