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

Leonardo Cavalcanti
Leonardo Cavalcanti
5,308 Points

generateHTML is called without passing any parameters but it has "data" for a parameter, how is the data being passed in

The function generateHTML has (data) for a parameter, but on line 44 it is being called as a callback inside the method getJSON without any parameters, how then is it functioning normally? In other words, how is the function generateHTML receiving its data for its generating the HTML without anything being given to it?

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);

// Generate the markup for each profile
function generateHTML(data) {
  const section = document.createElement('section');
  // Check if request returns a 'standard' page from Wiki
  if (data.type === 'standard') {
    section.innerHTML = `
      <img src=${data.thumbnail.source}>
  } else {
    section.innerHTML = `
      <img src="img/profile.jpg" alt="ocean clouds seen from space">
      <p>Results unavailable for ${data.title}</p>

btn.addEventListener('click', (event) => 
  getJSON(astrosUrl, (data) => {
    data.people.map(person => {
      getJSON(wikiUrl + person.name, generateHTML);

1 Answer

Steven Parker
Steven Parker
229,608 Points

The callback is being passed as an argument here, it is not being called (yet).
When it does get called from inside of the getJSON code (on line 13), it will be given the argument it needs.

Leonardo Cavalcanti
Leonardo Cavalcanti
5,308 Points

So the "return callback(data);" is invoked so the parameter "data" here becomes the parameter for generateHTML(data) ?