JSFiddle - React, Tailwind, and code Playground

JavaScript

let oldArea = [3, 4, 2]
let newArea = [3, 6, 2]

let changedArea = [];

      if (
        oldArea[0] !== newArea[0] &&
        oldArea[1] === newArea[1]
      ) {
        changedArea.push(newArea[0]);
      } else if (
        oldArea[1] !== newArea[1] &&
        oldArea[0] === newArea[0] &&
        oldArea[2] === newArea[2]
      ) {
        changedArea.push(newArea[0] + newArea[1]);
      } else changedArea.push(newArea.reduce((a, b) => a + b, 0));

      let changedAreaHeight = changedArea.reduce((a, b) => a + b, 0);

      console.log(changedAreaHeight)