Creating new customer

while creating new customer we can select Type(individual or company ).Based on our selection some extra fields appear .How is it possible? which js file? please help me

That’s done in the Customer doctype itself. You can check the Customer doctype in those field that appear after selecting individual or company - there’d be an option called ‘display depends on’ , just add a filter there like doc.customer_type!=Company and that field will then be only visible when customer type is not company.