books/models/baseModels/Party/PartyServer.js

import frappe from 'frappe';
import Document from 'frappe/model/document';

export default class PartyServer extends Document {
  beforeInsert() {
    if (this.customer && this.supplier) {
      throw new Error('Select a single party type.');
    }

    if (!this.customer && !this.supplier) {
      this.supplier = 1;
    }

    if (this.gstin && ['Unregistered', 'Consumer'].includes(this.gstType)) {
      this.gstin = '';
    }
  }

  async updateOutstandingAmount() {
    let isCustomer = this.customer;
    let doctype = isCustomer ? 'SalesInvoice' : 'PurchaseInvoice';

    const outstandingAmounts = (
      await frappe.db.getAllRaw(doctype, {
        fields: ['outstandingAmount', 'party'],
        filters: { submitted: true },
      })
    ).filter(({ party }) => party === this.name);

    const totalOutstanding = outstandingAmounts
      .map(({ outstandingAmount }) => frappe.pesa(outstandingAmount))
      .reduce((a, b) => a.add(b), frappe.pesa(0));

    await this.set('outstandingAmount', totalOutstanding);
    await this.update();
  }
}