fix: target es2018 to support node 10+

src/create-element.ts

@@ -5,6 +5,7 @@
 
 import { Component, Element, HTMLElement } from "./component";
 import { HTMLAttributes } from "./jsx";
+import { flatDeep } from "./utils";
 
 /**
  * Create an HTMLElement from a custom Component.
@@ -42,7 +43,7 @@ export function createElement(
     ...children: Element[]
 ): HTMLElement {
     // Flatten the children array so we can accept arrays as children.
-    const normalizedChildren = children.flat();
+    const normalizedChildren = flatDeep(children);
     if (type instanceof Function) {
         return type({ ...props, children: normalizedChildren });
     }

src/utils.ts

@@ -48,3 +48,26 @@ export const purgeModuleAndDepsFromCache = (modName: string): void => {
     }
     delete require.cache[modPath];
 };
+
+export type Flattenable<T> = Array<T | Flattenable<T>>;
+
+const flatDeepRec = <T>(arr: Flattenable<T>, d: number): T[] => {
+    if (d <= 0) {
+        return arr.slice() as T[];
+    }
+
+    let acc = [] as T[];
+    for (const val of arr) {
+        acc = acc.concat(Array.isArray(val) ? flatDeepRec(arr, d - 1) : val);
+    }
+    return acc;
+};
+
+/**
+ * Flatten an arbitrarily-deeply nested array into a flat array.
+ *
+ * @param arr Array to flatten.
+ *
+ * @return Flattened array.
+ */
+export const flatDeep = <T>(arr: Flattenable<T>): T[] => flatDeepRec(arr, 1);