build: add type support for older TypeScript versions

.gitignore

@@ -1,3 +1,4 @@
 node_modules
 dist/*
 types/*
+!types/legacy*

package.json

@@ -17,7 +17,11 @@
   },
   "license": "MIT",
   "main": "dist/cjs.js",
-  "types": "types/index.d.ts",
+  "types": "types/legacy/index.d.ts",
+  "typesVersions": {
+    "<4.1": { "*": ["types/legacy/*"] },
+    ">=4.1": { "*": ["types/*"] }
+  },
   "files": [
     "dist",
     "types",
@@ -30,6 +34,7 @@
     "build": "npm run build:js && npm run build:types",
     "build:js": "rollup -c",
     "build:types": "tsc -p tsconfig.typedef.json",
+    "postbuild:types": "echo \"// Minimum TypeScript Version: 4.1\n\n$(cat types/index.d.ts)\" > types/index.d.ts",
     "presize": "npm run build:js",
     "size": "terser --compress --mangle -- ./dist/umd.js | gzip -c | wc -c",
     "pretest": "npm run build:js",

types/legacy/index.d.ts

@@ -0,0 +1,30 @@
+/**
+ * Deeply merge two objects.
+ *
+ * @param target The first object.
+ * @param source The second object.
+ * @param options Deep merge options.
+ */
+export default function deepmerge<T1 extends object, T2 extends object>(target: T1, source: T2, options?: Options): T1 & T2;
+
+/**
+ * Deeply merge two or more objects.
+ *
+ * @param objects An tuple of the objects to merge.
+ * @param options Deep merge options.
+ */
+export function deepmergeAll<T1 extends object, T2 extends object>(objects: [T1, T2], options?: Options): T1 & T2;
+export function deepmergeAll<T1 extends object, T2 extends object, T3 extends object>(objects: [T1, T2, T3], options?: Options): T1 & T2 & T3;
+export function deepmergeAll<T1 extends object, T2 extends object, T3 extends object, T4 extends object>(objects: [T1, T2, T3, T4], options?: Options): T1 & T2 & T3 & T4;
+export function deepmergeAll<T1 extends object, T2 extends object, T3 extends object, T4 extends object, T5 extends object>(objects: [T1, T2, T3, T4, T5], options?: Options): T1 & T2 & T3 & T4 & T5;
+export function deepmergeAll<T extends object>(objects: ReadonlyArray<T>, options?: Options): T;
+
+/**
+ * Deep merge options.
+ */
+export type Options = {
+    arrayMerge?: (target: object[], source: object[], options: Options) => any;
+    clone?: boolean;
+    customMerge?: (key: string) => ((target: object, source: object, options: Options) => any) | undefined;
+    isMergeable?: (value: object) => boolean;
+}