What does .map do for an http?

I recommend that you use the latest version of angular , since link is already obsolete and now link is used.

Example:

Version ANGULAR <= 4.2

http is used to consume apis; and what .map does is transform the result into JSON to be used in your component with subscribe. (in this version of angular if or if you must use .map )

loadAll() {
    return this.http.get(this.url)
        .map(res => res.json());            
}

Version ANGULAR >= 4.3

httpClient is used to consume apis; in this version it is no longer necessary to use .map , since internally it transforms the result into json by default; and in your component you do the same consuming with subscribe.

loadAll() {
    return this.http.get(this.url);                   
}

I'll give you the official documentation how you should implement httpclient to your project: link

The behavior of .map in an observable is analogous to the behavior of .map in an array : Transform the elements into whatever you want using the function you pass.

For example, let's say you have an http call like your example:

 return this.http.get(this.url)
    .map(res => res.json());

I assume that this.http is an attribute of the class Http , so I warn you that its use is already considered outdated .

What happens here is that the call this.http.get(url) returns a Observable<Response> , that is, you have a Response object. But what you are interested in is the answer itself, a JSON string with your data. Therefore, with .map(...) you make a transformation: each object of type Response emitted is transformed into an object resulting from doing JSON.parse (...) to the text of the response.

I show you an example of how it works with an array:

let arrayTexto=['100','99'];

let transformaaNumeroYSumaCinco=function (texto) {
  return +texto + 5;
}

console.log(arrayTexto.map(transformaaNumeroYSumaCinco));

Now, as I mentioned before, this transformation is necessary because the Http.get method returns a Response, but it is considered outdated because in Angular v4.3 and later there is a class HttpClient that also has the methods get, post , put ... and directly return an object already parsed .