CODEWITHSUNDEEP
javascriptnode.jsmongodbmongoose
Frank Matranga
Frank Matranga

Reputation: 184

Why does a object claim to be undefined only when I try to acces one of its properties?

With the following code:

req.Course.find({}).populate('students', 'username firstName lastName registered').populate('teacher', 'image mID firstName lastName').sort({title: 1}).exec(function(err, courses){
    if(err) throw err;
    req.toJade.courses = [];
    if (courses){
        req.toJade.courses = advs;
    }
    res.render('courses/list', req.toJade);

});

courses is an array of objects in the following format:

[{
  _id: objectid,
  title: 'History',
  teacher:
   {
     _id: objectid,
     name: 'John Smith',
     etc...   
   },
   etc...
},
etc...
]

I loop through the array of those objects in Jade with and when I try to access a property of any teacher with

for c in courses:
    h1= c.title
        small= c.teacher.name

it returns an error that it cannot read property 'name' of undefined. And this occurs when I try to access any other property of the course object.

However, using

for c in courses:
    h1= c.title
        small= c.teacher

prints out the whole clearly defined object with the all of its properties existing and with the correct values.

What am I doing wrong and how can I access any of the properties of the teacher objects?

Upvotes: 1

Views: 94

Answers (3)

Tibos
Tibos

Reputation: 27843

for..in iterates over all properties of an object. In the case of Arrays, this means all of its values, all of the properties set on the array itself and all of the properties set on Array.prototype. Usually when dealing with Arrays you are only interested in the values, so for..in is not the best choice.

The proper way to iterate over Array values is by using a a simple for/while/do while or by using Array#forEach and other similar functional iteration methods:

for (var i=0; i<courses.length; i++) {
  var c = courses[i];
  h1 = c.title;
  small = c.teacher.name;
}
// or
courses.forEach(function(c){
  h1 = c.title;
  small = c.teacher.name;
});

Also, a simple wayto avoid the error if the teacher property is not set would be:

small = c.teacher && c.teacher.name;

If c.teacher is undefined, small will be undefined as well, otherwise it will be equal to c.teacher.name.

Adding a default value is just as easy:

small = c.teacher && c.teacher.name || 'Unknown teacher';

Upvotes: 2

Varatharaj
Varatharaj

Reputation: 687

java script will check for the existence of properties for a particular object only when we access it. Because it is not typed language as C#,Java. In java script we can add or remove properties dynamically.

To check for the existence of a property in an object we can use the function which is available for all the objects in javascript.

objectname.hasOwnProperty("propertyToCheck")

Upvotes: 2

Frank Matranga
Frank Matranga

Reputation: 184

I have found my dumb mistake. I wasn't expecting 2 objects in the courses array to have their teacher as undefined. I guess that broke the whole thing since I never checked to see if the teacher property exists.

Upvotes: 1

Related Questions