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JavaScript Asynchronous Programming with JavaScript Asynchronous JavaScript with Callbacks Implement a Callback

Hamzah Iqbal
Hamzah Iqbal
Full Stack JavaScript Techdegree Student 11,145 Points

How does the generateHTML work, when no functions are passed to it?

function generateHTML(data) {
  const section = document.createElement('section');
  section.innerHTML = `
    <img src=${data.thumbnail.source}>

btn.addEventListener("click", () => {

getJSON(astrosUrl, (json) =>


 person.people.map(person=> {

   getJSON(wikiUrl + person.name, generateHTML);


I can somewhat see the reason for the generateHTML, however I cant see, why no data is passed in as the argument here. How is the data passed in here?

1 Answer

Steven Parker
Steven Parker
210,147 Points

In the "getJSON" call shown here, only a reference to "generateHTML" is being passed as a callback. It's not being called yet.

But if you go look at the definition for "getJSON" (not shown here), you can see that when it does get called (at line 13 of "callback.js"),, it is passed a data argument ("callback(data)").