JSFiddle - React, Tailwind, and code Playground

JavaScript

function getTwoDimention(input){
    var input = input.split('\n');
    var twoDimArr=[];
    for(var n=0; n<input.length; n++){ 
        var subarr = input[n].split(' ');
       if(subarr.length > 1){
           twoDimArr.push(subarr)
       }
    }
    return twoDimArr;
}

function getFristDiagonal(twoDimArr){
    var sum = 0;
    for(var i=0; i<twoDimArr.length; i++){
        for(var j=i; j<=i; j++){
           sum += parseFloat(twoDimArr[i][j]);
        }
    }
    return sum;
}
function getSecondDiagonal(twoDimArr){
    var sum = 0;j=twoDimArr.length-1;
    for(var i=0; i<twoDimArr.length; i++){
        
           sum += parseFloat(twoDimArr[i][j--]);
    }
    return sum;
}
function processData(input) {
    //Enter your code here
    twoDimArr = getTwoDimention(input);
    var firtDia = getFristDiagonal(twoDimArr);
    var secDia = getSecondDiagonal(twoDimArr);
    
    console.log(secDia - firtDia);
    
} 
var input = 
    "3\n"+
"11 2 4\n"+
"4 5 6\n"+
"10 8 -12\n";
processData(input) ;