ReactJS toLowerCase is not a function

Question

I am doing a comparison by converting my text to lowercase and comparing its index to -1, in order to have some value to the particular field in ReactJS, but I am getting this error in JavaScript console:

Uncaught TypeError: props.filterText.toLowerCase is not a function

var props = this.props;
var rows = this.props.episodes
    .filter(function(episode){
        return episode.title.toLowerCase().indexOf(props.filterText.toLowerCase()) > -1;
    })
    .map(function(episode){
       return <EpisodeRow key={episode.title} episode={episode}/>
    });

Answer 1

Looks like Chris's comment is the correct answer:

If title is a string, use toString() before toLowerCase():

return episode.title.toString().toLowerCase().indexOf(props.filterText.toString().toLowerCase()) > -1;

Answer 2

I think Adam's answer from the comments is actually correct, and turned out to solve my problem :

.filter(function(episode){
    if(episode.title) {
        return episode.title.toLowerCase().indexOf(props.filterText.toLowerCase()) > -1;
    } else {
        return '';
    }
})

Answer 3

I think I found the problem :)

props.filterText is undefined. => The filterText:"" must be declared in the state before putting in props