FIFO in Javascrip
<script>
    var purchase = [{
        "qty": 100,
        "price": 200,
        "amt": 0
      },
      {
        "qty": 275,
        "price": 210,
        "amt": 0
      },
      {
        "qty": 300,
        "price": 225,
        "amt": 0
      },
      {
        "qty": 500,
        "price": 275,
        "amt": 0
      },
    ];
    var sells = [{
        "qty": 600,
        "amt": 0,
        "purAmt": 0,
        "price": 300
      },
      {
        "qty": 275,
        "amt": 0,
        "purAmt": 0,
        "price": 350
      },
      {
        "qty": 300,
        "amt": 0,
        "purAmt": 0,
        "price": 400
      }
    ];
    purchase.forEach((element, key) => {
      purchase[key]['amt'] = element['qty'] * element['price'];
    });
    sells.forEach((sell, sell_key) => {
      var purAmount = 0;
      var balance = sell['qty'];
      var usedQty = 0;
      var detail = [];
      purchase.forEach((pur, pur_key) => {
        if (balance != 0) {
          if (pur['qty'] <= balance) {
            purAmount += pur['qty'] * pur['price'];
            detail.push(pur['qty'] + "*" + pur['price'] + "=" + pur['qty'] * pur['price'])
            balance -= pur.qty;
            purchase[pur_key]['qty'] = 0;
            delete purchase[pur_key];

          } else {
            if (pur['qty'] > balance) {
              balance -= pur['qty'];
              usedQty = pur['qty'] - Math.abs(balance);
              purchase[pur_key]['qty'] = Math.abs(balance);
              if (balance != 0) {
                purAmount += usedQty * pur['price'];
                detail.push(usedQty + "*" + pur['price'] + "=" + usedQty * pur['price']);
              }
              balance = Math.max(0, balance);
              return false;
            }
          }
        }
      });
      sells[sell_key]['details'] = [...detail];
      sells[sell_key]['purAmt'] = purAmount;
      sells[sell_key]['amt'] = sells[sell_key]['qty'] * sells[sell_key]['price'];
      sells[sell_key]['gainLoss'] = sells[sell_key]['amt'] - sells[sell_key]['purAmt'];
    });
    console.log(sells);
    console.log(purchase);