Create convert-callback-based-function-to-promise-based-function.ts

Author: kamyu104

TypeScript/convert-callback-based-function-to-promise-based-function.ts

@@ -0,0 +1,20 @@
+// Time:  O(1)
+// Space: O(1)
+
+type CallbackFn = (
+    next: (data: number, error: string) => void, 
+    ...args: number[]
+) => void
+type Promisified = (...args: number[]) => Promise<number>
+
+// promise
+function promisify(fn: CallbackFn): Promisified {
+    return async function(...args) {
+        return new Promise((resolve, reject) => {
+            const cb = (val, err) => {
+                err ? reject(err) : resolve(val);
+            };
+            fn(cb, ...args);
+        });
+    };
+};