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JavaScript Asynchronous Programming with JavaScript Asynchronous JavaScript with Callbacks Managing Nested Callbacks

Managing nested callbacks

const astrosUrl = 'http://api.open-notify.org/astros.json';
const wikiUrl = 'https://en.wikipedia.org/api/rest_v1/page/summary/';
const peopleList = document.getElementById('people');
const btn = document.querySelector('button');

// Make an AJAX request
function getJSON(url, callback) {
  const xhr = new XMLHttpRequest();
  xhr.open('GET', url);
  xhr.onload = () => {
    if(xhr.status === 200) {
      let data = JSON.parse(xhr.responseText);
      return callback(data);

function getProfiles(json) {
  json.people.map( person => {
    getJSON(wikiUrl + person.name, generateHTML);

// Generate the markup for each profile
function generateHTML(data) {
  const section = document.createElement('section');
  section.innerHTML = `
    <img src=${data.thumbnail.source}>

btn.addEventListener('click', (event) => { 
  getJSON(astrosUrl, getProfiles);

In this code, where getProfile function has been declared, json.people.map calls a callback function with parameter person and that person parameter then accesses the name property of the people object. My question is that person property has not been declared inside the people object so how this function uses this to name property and then links it to name? thanks.

1 Answer

Steven Parker
Steven Parker
210,487 Points

Since JavaScript is a loosely-typed dynamic language, a property can be recognized by name in an object passed into a parameter. Unless you are creating a new object, there's no "declaration" process needed to identify the attributes of an object.

For more details, see the MDN page on JavaScript data types and data structures.

Zoltán Erkel
Zoltán Erkel
6,328 Points

Just wondering, if also the arrow function actually declares the "person" as the parameter of the object? I mean the declaration actually happens when we use the arrow function