Why is argument missing in chained Map operator

问题

I'm getting to know hyperledger composer angular apps. This piece of code that yeoman generator produces puzzle me. The add method executes return this.http.post and it returns a response object, that is chained to a map operator to convert to json output. But all references I could find to map operator show that its argument must be specified, like an arrow function (v)=> {some instructions to perform on v here});, or as an anonymous function (f(v){some instructions here});

I've seem the code below also in tutorials as a best practice to deal with response objects. Obviously map operator also accepts a function and it sends the resulting response object to whom is chained to as the argument of the `map function. Could anyone provide links to where this mechanism is explained?

import { Injectable } from '@angular/core';
import { Http, Response, Headers } from '@angular/http';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/catch';


public add(ns: string, asset: Type): Observable<Type> {
    console.log('Entered DataService add');
    console.log('Add ' + ns);
    console.log('asset', asset);
    return this.http.post(this.actionUrl + ns, asset)
            .map(this.extractData)
            .catch(this.handleError);
}


private extractData(res: Response): any {
    return res.json();
}

来源：https://stackoverflow.com/questions/50648218/why-is-argument-missing-in-chained-map-operator