functional: add undefined checker

src/functional.ts

@@ -6,3 +6,11 @@ export const sequence = <T, U>(fn: ($: T) => Promise<U>) => async (L: T[]) => {
 export const parallel = <T, U>(fn: ($: T) => Promise<U>) => async (L: T[]) => {
     await Promise.all(L.map(fn));
 };
+
+
+export const notUndefined: <T>($: T | undefined) => T = $ => {
+    if ($ === undefined)
+        throw Error('the value is undefined!');
+    else
+        return $;
+};