[SharePoint]javascript client object model 获取lookup 类型的field的值，包括user类型（单人或者多人）的值。how to get the multiple user type/lookup type field value by Javascript client object model

1. how to get value

var context = new SP.ClientContext.get_current();
var web = context.get_web();
var list = web.get_lists().getByTitle(listTitle);
var listItem = list.getItemById(1);   
context.load(listItem);
context.executeQueryAsync(
   function() {
       var lookupVals = listItem.get_item(fieldName); //get multi lookup value (SP.FieldLookupValue[])
       for(var i = 0;i < lookupVals.length;i++) {
           console.log(lookupVals[i].get_lookupId()); //print Id
           console.log(lookupVals[i].get_lookupValue()); //print Value
       }
   },
   function(sender,args){
       console.log(args.get_message());
   }
);

2. how to update value

var context = new SP.ClientContext.get_current();
var web = context.get_web();
var list = web.get_lists().getByTitle(listTitle);
var listItem = list.getItemById(1);   

var lookupVals = [];
//set 1st Lookup value
var lookupVal1 = new SP.FieldLookupValue();
lookupVal1.set_lookupId(1);
lookupVals.push(lookupVal1);
//set 2nd Lookup value
var lookupVal2 = new SP.FieldLookupValue();
lookupVal2.set_lookupId(2);
lookupVals.push(lookupVal2);

listItem.set_item(fieldName,lookupVals);
listItem.update();

context.executeQueryAsync(
   function() {
        console.log('Multi lookup field has been updated');
   },
   function(sender,args){
       console.log(args.get_message());
   }
);