How to get sum of field from child table

[rmehta]

frappe.ui.form.on("quotesection", "width", function(frm, cdt, cdn){
  var d = locals[cdt][cdn];
  frappe.model.set_value(d.doctype, d.name, "sqft", d.length * d.width);

  var total = 0;
  frm.doc.quotesection.forEach(function(d) { total += d.sqft; });

  frm.set_value('total_sqft', total);

});