Currency API - how to use them

I know there must be API to convert currencies hidden somewhere.
How can I access them?

David

frappe.call({
method: “erpnext.setup.utils.get_exchange_rate”,
args: {
from_currency: “USD”,
to_currency: “MMK”,
transaction_date:“2017-04-01”
},
callback: function( r ) {
console.log(flt(r.message));
}
});

Thank you so much!