What are async iterators and how are they different from regular iterators?

 const syncIterable = {
   [Symbol.iterator]() {
     let i = 0;
     return {
       next() {
         if (i < 3) {
           return { value: i++, done: false };
         }
         return { done: true };
       }
     };
   }
 };

 for (const val of syncIterable) {
   console.log(val); // 0, 1, 2
 }

 const asyncIterable = {
   [Symbol.asyncIterator]() {
     let i = 0;
     return {
       async next() {
         if (i < 3) {
           await new Promise(resolve => setTimeout(resolve, 1000));
           return { value: i++, done: false };
         }
         return { done: true };
       }
     };
   }
 };

 (async () => {
   for await (const val of asyncIterable) {
     console.log(val); // 0, 1, 2 (one per second)
   }
 })();

 async function* fetchPages(urls) {
   for (const url of urls) {
     const response = await fetch(url);
     const data = await response.json();
     yield data;
   }
 }

 (async () => {
   const urls = ['api/page1', 'api/page2', 'api/page3'];
   for await (const page of fetchPages(urls)) {
     console.log(page);
   }
 })();

 async function* readLines(filePath) {
   const fileStream = fs.createReadStream(filePath);
   const rl = readline.createInterface({
     input: fileStream,
     crlfDelay: Infinity
   });

   for await (const line of rl) {
     yield line;
   }
 }

 (async () => {
   for await (const line of readLines('large-file.txt')) {
     console.log(line);
   }
 })();