Parallel URL Content Download with JavaScript Promises

JavaScript Asynchronous: Exercise-4 with Solution

Parallel URL Downloads with Promises

Write a JavaScript function that takes an array of URLs and downloads the contents of each URL in parallel using Promises.

Sample Solution:

JavaScript Code:

function downloadContents(urls) {
  const promises = urls.map(url => {
    return new Promise((resolve, reject) => {
      fetch(url)
        .then(response => {
          if (!response.ok) {
            throw new Error(`HTTP error! Status: ${response.status}`);
          }
          return response.text();
        })
        .then(data => resolve(data))
        .catch(error => reject(error));
    });
  });

  return Promise.all(promises);
}

// Usage example:
const urls = [
  'https://jsonplaceholder.typicode.com/posts/1',
  'https://jsonplaceholder.typicode.com/posts/2',
  'https://jsonplaceholder.typicode.com/posts/3'
];

downloadContents(urls)
  .then(contents => {
    console.log('Downloaded contents:', contents);
  })
  .catch(error => {
    console.log('Error:', error.message);
  });

Output:

"Downloaded contents:"
["{
  \"userId\": 1,
  \"id\": 1,
  \"title\": \"sunt aut facere repellat provident occaecati excepturi optio reprehenderit\",
  \"body\": \"quia et suscipit\nsuscipit recusandae consequuntur expedita et cum\nreprehenderit molestiae ut ut quas totam\nnostrum rerum est autem sunt rem eveniet architecto\"
}", "{
  \"userId\": 1,
  \"id\": 2,
  \"title\": \"qui est esse\",
  \"body\": \"est rerum tempore vitae\nsequi sint nihil reprehenderit dolor beatae ea dolores neque\nfugiat blanditiis voluptate porro vel nihil molestiae ut reiciendis\nqui aperiam non debitis possimus qui neque nisi nulla\"
}", "{
  \"userId\": 1,
  \"id\": 3,
  \"title\": \"ea molestias quasi exercitationem repellat qui ipsa sit aut\",
  \"body\": \"et iusto sed quo iure\nvoluptatem occaecati omnis eligendi aut ad\nvoluptatem doloribus vel accusantium quis pariatur\nmolestiae porro eius odio et labore et velit aut\"
}"]

Note: Executed on JS Bin

Explanation:

In the above exercise -

• The "downloadContents()" function accepts an array of URLs as the urls parameter.
• It creates an array of Promises using the "map()" method on the urls array. Each Promise represents a fetch request to a specific URL.
• Inside each Promise, a fetch request is made to the corresponding URL using the "fetch()" function. It returns a Promise that resolves with the response.
• The first .then block checks if the response is successful (response.ok). If not, it throws an error with a custom message indicating HTTP status.
• If the response is successful, the second .then block extracts the response data using response.text(), which returns a Promise that resolves with the text content.
• Resolved data is then passed to the resolve function to fulfill the Promise.
• If any error occurs during the fetch request or response handling, the .catch block is executed. Errors are passed to the reject function, which rejects the Promise.
• The resulting array of Promises is returned from the downloadContents function using Promise.all, which returns a Promise that resolves to an array of fulfilled Promise values.

In the usage example, an array of URLs is defined, representing three different posts from the jsonplaceholder.typicode.com API.

• The downloadContents function is called with the urls array, initiating the parallel download of contents from the specified URLs.
• When all the Promises are fulfilled (i.e., when all the contents are downloaded), the .then block is executed, and the downloaded contents are logged to the console.
• If any error occurs during the process, the .catch block is executed, and the error message is logged to the console.

Flowchart:

Live Demo:

See the Pen javascript-asynchronous-exercise-4 by w3resource (@w3resource) on CodePen.

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