[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());
    }
 );