db.student.find().forEach(function(x){
 //under normal circumstances, the field to obtain the Document is as follows
 //Get Student Names
 print(x.stuName);
 //Obtain student age
 print(x.stuAge);
 });
 
 //But I now have a requirement that this fieldName is dynamic.
 //For example, I need to obtain different field values according to the passed fieldName.
 //For example, I passed the string "stub" when calling this function
 function handleField(fieldName){
 db.student.find().forEach(function(x){
 //Print out stuName
 print(fieldName);
 //print out undefined
 print(x.fieldName);
 });
 }
 
 Why the second method is not successful, is it an x.fieldName statement, this fieldName can only specify dead, and cannot obtain the corresponding field according to the dynamically passed-in value.