fix: add legacy type information

package.json

@@ -38,7 +38,19 @@
   },
   "main": "lib/index.cjs",
   "module": "lib/index.mjs",
-  "types": "types/current/index.d.ts",
+  "types": "types/legacy/v4_0.d.ts",
+  "typesVersions": {
+    "<4.1": {
+      "*": [
+        "types/legacy/v4_0.d.ts"
+      ]
+    },
+    ">=4.1": {
+      "*": [
+        "types/current/index.d.ts"
+      ]
+    }
+  },
   "files": [
     "lib/",
     "types/",
@@ -122,14 +134,6 @@
     "tslib": "^2.3.1",
     "typescript": "^4.4.3"
   },
-  "peerDependencies": {
-    "typescript": ">=4.1.0"
-  },
-  "peerDependenciesMeta": {
-    "typescript": {
-      "optional": true
-    }
-  },
   "engines": {
     "node": ">=12.4.0"
   }

tsconfig.base.json

@@ -29,6 +29,7 @@
     },
   },
   "exclude": [
-    "benchmark/"
+    "benchmark/",
+    "types/"
   ]
 }

types/legacy/v4_0.d.ts

@@ -0,0 +1,75 @@
+/**
+ * @file
+ *
+ * These types are a more simplified version of this library's types
+ * designed to work with versions of TypeScript < 4.1
+ */
+
+/**
+ * All the options the user can pass to customize deepmerge.
+ */
+declare type DeepMergeOptionsFull = Readonly<{
+  mergeRecords: DeepMergeMergeFunctions["mergeRecords"] | false;
+  mergeArrays: DeepMergeMergeFunctions["mergeArrays"] | false;
+  mergeMaps: DeepMergeMergeFunctions["mergeMaps"] | false;
+  mergeSets: DeepMergeMergeFunctions["mergeSets"] | false;
+  mergeOthers: DeepMergeMergeFunctions["mergeOthers"];
+}>;
+
+/**
+ * All the merge functions that deepmerge uses.
+ */
+declare type DeepMergeMergeFunctions = Readonly<{
+  mergeRecords: <Ts extends ReadonlyArray<Readonly<Record<keyof any, any>>>, U extends DeepMergeMergeFunctionUtils>(records: Ts, utils: U) => any;
+  mergeArrays: <Ts extends ReadonlyArray<Readonly<ReadonlyArray<any>>>, U extends DeepMergeMergeFunctionUtils>(records: Ts, utils: U) => any;
+  mergeMaps: <Ts extends ReadonlyArray<Readonly<ReadonlyMap<any, any>>>, U extends DeepMergeMergeFunctionUtils>(records: Ts, utils: U) => any;
+  mergeSets: <Ts extends ReadonlyArray<Readonly<ReadonlySet<any>>>, U extends DeepMergeMergeFunctionUtils>(records: Ts, utils: U) => any;
+  mergeOthers: <Ts extends ReadonlyArray<any>, U extends DeepMergeMergeFunctionUtils>(records: Ts, utils: U) => any;
+}>;
+
+/**
+ * The utils provided to the merge functions.
+ */
+declare type DeepMergeMergeFunctionUtils = Readonly<{
+  mergeFunctions: DeepMergeMergeFunctions;
+  defaultMergeFunctions: DeepMergeMergeFunctionsDefaults;
+  deepmerge: <Ts extends ReadonlyArray<any>>(...values: Ts) => any;
+}>;
+
+/**
+ * The default merge functions.
+ */
+declare type DeepMergeMergeFunctionsDefaults = Readonly<{
+  mergeMaps: (values: Record<keyof any, any>[], utils: DeepMergeMergeFunctionUtils) => any;
+  mergeSets: (values: any[][], utils: DeepMergeMergeFunctionUtils) => any;
+  mergeArrays: (values: Set<any>[], utils: DeepMergeMergeFunctionUtils) => any;
+  mergeRecords: (values: Map<any, any>[], utils: DeepMergeMergeFunctionUtils) => any;
+  mergeOthers: (values: any[], utils: DeepMergeMergeFunctionUtils) => any;
+}>;
+
+/**
+ * Deeply merge objects.
+ *
+ * @param objects - The objects to merge.
+ */
+declare function deepmerge(): undefined;
+declare function deepmerge<T0>(arg0: T0): T0;
+declare function deepmerge<T0, T1>(arg0: T0, arg1: T1): T0 & T1;
+declare function deepmerge<T0, T1, T2>(arg0: T0, arg1: T1, arg2: T2): T0 & T1 & T2;
+declare function deepmerge<T0, T1, T2, T3>(arg0: T0, arg1: T1, arg2: T2, arg3: T3): T0 & T1 & T2 & T3;
+declare function deepmerge<T0, T1, T2, T3, T4>(arg0: T0, arg1: T1, arg2: T2, arg3: T3, arg4: T4): T0 & T1 & T2 & T3 & T4;
+declare function deepmerge<T0, T1, T2, T3, T4, T5>(arg0: T0, arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5): T0 & T1 & T2 & T3 & T4 & T5;
+declare function deepmerge<T0, T1, T2, T3, T4, T5, T6>(arg0: T0, arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5, arg6: T6): T0 & T1 & T2 & T3 & T4 & T5 & T6;
+declare function deepmerge<T0, T1, T2, T3, T4, T5, T6, T7>(arg0: T0, arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5, arg6: T6, arg7: T7): T0 & T1 & T2 & T3 & T4 & T5 & T6 & T7;
+declare function deepmerge<T0, T1, T2, T3, T4, T5, T6, T7, T8>(arg0: T0, arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5, arg6: T6, arg7: T7, arg8: T8): T0 & T1 & T2 & T3 & T4 & T5 & T6 & T7 & T8;
+declare function deepmerge<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9>(arg0: T0, arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5, arg6: T6, arg7: T7, arg8: T8, arg9: T9): T0 & T1 & T2 & T3 & T4 & T5 & T6 & T7 & T8 & T9;
+declare function deepmerge(...args: any[]): any;
+
+/**
+ * Deeply merge two or more objects using the given options.
+ *
+ * @param options - The options on how to customize the merge function.
+ */
+declare function deepmergeCustom(options: Partial<DeepMergeOptionsFull>): (...objects: any[]) => any;
+
+export { deepmerge, deepmergeCustom };