js-concepts: parallel programming - Symbol.interator/Symbol.asyncIterator

Author: baochau.dinh

Parallel/README.md

@@ -1,4 +1,67 @@
 - `Promise.race()` lets you know as soon as one of the given promises either fulfills or rejects
 - `Promise.allSettled()` lets you know when all promises either fulfill or reject
 - `Promise.all()` lets you know as soon as one of the given promises rejects or when all of them fulfill
-- `Promises.any()` lets you know as soon as one of the given promises fulfills or when none of them fulfills.
+- `Promises.any()` lets you know as soon as one of the given promises fulfills or when none of them fulfills.
+
+## Creating custom iterator
+
+`[Symbol.iterator]()` works like any other method except that it's automatically called if we use `for...of`
+on the object. 
+
+The iterator object is designed to maintain internal pointer to a position in the items.
+
+```JS
+const collection = {
+  a: 10,
+  b: 20,
+  c: 30,
+  [Symbol.iterator]() {
+    let i = 0;
+    const values = Object.keys(this);
+    return {
+      next: () => {
+        return {
+          value: this[values[i++]],
+          done: i > values.length
+        }
+      }
+    }
+  }
+}
+
+for (const value of collection) {
+  console.log(value);
+}
+```
+
+## Creating custom asynchronous iterator
+
+An object is classified as asychronous iterable when it has a `Symbol.asyncIterator` method that returns an 
+asynchronous iterator.
+
+```JS
+const collection = {
+  a: 10,
+  b: 20,
+  c: 30,
+  [Symbol.asyncIterator]() {
+    const keys = Object.keys(this);
+    console.log('keys: ', keys);
+    let i = 0;
+    return {
+      next: () => {
+        return new Promise((resolve, reject) => {
+          setTimeout(() => {
+            resolve({
+              value: this[keys[i++]],
+              done: i > keys.length
+            });
+          }, 1000);
+        });
+      }
+    };
+  }
+};
+
+const iterator = collection[Symbol.asyncIterator]();
+```

Parallel/iteratorsV2.js

@@ -0,0 +1,21 @@
+const collection = {
+  a: 10,
+  b: 20,
+  c: 30,
+  [Symbol.iterator]() {
+    let i = 0;
+    const values = Object.keys(this);
+    return {
+      next: () => {
+        return {
+          value: this[values[i++]],
+          done: i > values.length
+        }
+      }
+    }
+  }
+}
+
+for (const value of collection) {
+  console.log(value);
+}

Parallel/workWithMultipleApis.js

@@ -0,0 +1,49 @@
+/**
+ * Retrieving URLs separately
+ */
+
+import fetch from 'node-fetch';
+
+const srcArr = [
+  'https://eloux.com/async_js/examples/1.json',
+  'https://eloux.com/async_js/examples/2.json',
+  'https://eloux.com/async_js/examples/3.json',
+];
+
+srcArr[Symbol.asyncIterator] = function() {
+  let i = 0;
+  return {
+    async next() {
+      if (i === srcArr.length) {
+        return {
+          done: true
+        }
+      } else {
+        const url = srcArr[i++];
+        const response = await fetch(url);
+        if (!response.ok) {
+          throw new Error('Unable to retrieve URL: ' + url);
+        }
+
+        return {
+          value: await response.json(),
+          done: false
+        }
+      }
+    }
+  }
+}
+
+const iterator = srcArr[Symbol.asyncIterator]();
+
+iterator.next().then(result => {
+  console.log(result.value.firstName);
+});
+
+iterator.next().then(result => {
+  console.log(result.value.firstName);
+});
+
+iterator.next().then(result => {
+  console.log(result.value.firstName);
+});

package.json

@@ -27,5 +27,6 @@
   "dependencies": {
     "async": "^3.2.3",
     "node-fetch": "^3.2.0"
-  }
+  },
+  "type": "module"
 }