Why do I get Cannot read property 'toString' of undefined

Question

I use this package. I've added these console.log's at the beginning of the slug function.

function slug(string, opts) {
    console.log('log 1: -------');
    console.log('log 2: ' + string);
    console.log('log 3: ' + typeof string);
    string = string.toString();
    ....
 }

Here is the output:

log 1: -------
log 2: Yüzey Aktif Maddeler
log 3: string
/Users/------/seeder/node_modules/mongodb/lib/utils.js:99
    process.nextTick(function() { throw err; });
                                  ^

TypeError: Cannot read property 'toString' of undefined
    at slug (/Users/------/seeder/node_modules/slug/slug.js:16:20)
    at Object.createSlug (/Users/------/seeder/seeder/utils.js:4:9)
    at getContent (/Users/------/seeder/seeder/content-seeder.js:345:22)
    at topic.contents.forEach.content (/Users/------/seeder/seeder/content-seeder.js:425:31)
    at Array.forEach (native)
    at /Users/------/seeder/seeder/content-seeder.js:424:21
    at /Users/------/seeder/node_modules/mongodb/lib/collection.js:523:5
    at /Users/------/seeder/node_modules/mongodb/lib/collection.js:701:5
    at handleCallback (/Users/------/seeder/node_modules/mongodb/lib/utils.js:96:56)
    at executeCommands (/Users/------/seeder/node_modules/mongodb/lib/bulk/ordered.js:405:12)

Even if type of string parameter is a string, string.toString() throws a TypeError. It says string is undefined.

I'm not an experienced js developer. Why would this happen?

Edit: I call it from this function

function getContent(content, categoryId) {
    console.log(content.topicVideo);
    return {
        // _id: Random.id(),
        // other properties
        contentSlug: slug(content.topicVideo.contentName, {
            lower: true
        })
    };
}

New logs:

{ id: '197505065',
  categorySort: 18,
  categoryName: 'Hayatımızda Kimya',
  contentSort: 2,
  contentName: 'Yüzey Aktif Maddeler',
  fileName: 'KonuAnlatimi.mp4' }

Edit:

I've added a log at the end of the slug function.

    console.log(result);
    return result;
};

This prints the results and then throws error.

log 1: -------
log 2: Yüzey Aktif Maddeler
log 3: string
log 4: Yüzey Aktif Maddeler
yuzey-aktif-maddeler

Edit: I think this line gives a hint:

/Users/------/seeder/node_modules/mongodb/lib/utils.js:99
    process.nextTick(function() { throw err; });

I think it logs correct string, but fails on undefined one without logging the logs. Is this possible? I've found out that one of content.topicVideo.contentName is actually undefined. But error is thrown without logging it. Is this even possible?

Answer 1

To make sure that .toString() doesn't throw an error, you could:

string = "" + string; // amends the value to an string, even if its undefined or null etc

instead of:

string = string.toString();

However you could also check if the value of string is undefined or null. Also see here.