Create 08. Promise.md

Author: ps0305

Machine Coding/08. Promise.md

@@ -0,0 +1,49 @@
+### Implement a Promise that takes a function following the common callback-last error-first style.
+
+
+```js
+/**
+ * Converts a callback-based function into a function that returns a Promise.
+ *
+ * @param {Function} func - The function to promisify (follows callback-last, error-first convention).
+ * @returns {Function} - A new function that returns a Promise.
+ */
+export default function promisify(func) {
+  if (typeof func !== 'function') {
+    throw new TypeError('Expected a function');
+  }
+
+  return function (...args) {
+    // Return a new Promise
+    return new Promise((resolve, reject) => {
+      // Call the original function with an additional callback
+      func(...args, (err, result) => {
+        if (err) {
+          // Reject the promise if there's an error
+          reject(err);
+        } else {
+          // Resolve the promise with the result
+          resolve(result);
+        }
+      });
+    });
+  };
+}
+```
+
+### Explanation
+#### Validate Input:
+
+* Check if the input `func` is a valid function. If not, throw a `TypeError`.
+#### Return a New Function:
+
+* The returned function preserves the original function's signature (i.e., it returns a function that takes any number of arguments).
+#### Create a Promise:
+
+* Inside the returned function, we wrap the execution in a Promise.
+* The callback-based function `(func)` is called with all the arguments spread (`...args`), and we append a custom callback as the last argument.
+#### Handle the Callback:
+
+* The custom `callback` follows the error-first convention ((err, result)):
+* If err exists (i.e., the operation failed), reject the Promise with the error.
+* Otherwise, resolve the Promise with the result.