Declaration error (this)

Ionic Framework ionic-v3
3

I moved the function into the class, that works.
By reading this message, I had to change the structure of the http.post, I replaced success by subscribe and I deleted error, I put this into my code :

let m_url=url_autentication2;
let m_data=soapRequest();
let response_soap='';
this.http.post(m_url,m_data)
	   .subscribe(m_data=>{response_soap = readBody(m_data);});
alert(response_soap);

There is no execution error but nothing is returned, I will see that with a workmate as I think the prob comes from the web service itself