To add or update customer fields, please do the following, call the method passing a JSONArray containing a JSONObject:
Customer.assignOrUpdateValueToCustomerFields({
fields: { "field1": "field1_value", "field2": "field2_value" }
}, (customer) => {
// Do something
}, (error) => {
// Log error
})
Otherwise, if you want to eliminate a field, you have to do this with an array of strings containing the name of the fields:
Customer.deleteValuesFromCustomerFields(["field1", "field2"], (customer) => {
// Do something
}, (error) => {
// Log error
})