Can I use a function expression as class method in javascript?

Question

I am looking forward about a concrete question in javascript.

I would like to use a function expression as a class method, but I don´t know if it is possible. I try to look any documentation about it but I didn´t find... This is an example:

class TourIdentifier extends HttpClient {
   constructor(baseURL, langCode, ...attr) {
       super({
            baseURL,
            headers: {
            lang: langCode
       }
       });

      this.model.init(...attr);
    }

    model =  function init(...attr) {
    this.tours_identifiers_identifier   = attr[0];
    this.tours_id                       = attr[1];
    this.lang_code                      = attr[2];
    this.tours_id_                      = attr[3];
    this.tours_identifiers_id           = attr[4];
    }

...

I know there are many ways to do this, but I just have this question, and I hope that anyone with more experience than me in JS can share they knowledge.

I appreciate your comments!

Best regards!