Move internal modules to “internal” folder.

.internal/createMathOperation.js

@@ -0,0 +1,38 @@
+import baseToNumber from './.internal/baseToNumber.js';
+import baseToString from './.internal/baseToString.js';
+
+/**
+ * Creates a function that performs a mathematical operation on two values.
+ *
+ * @private
+ * @param {Function} operator The function to perform the operation.
+ * @param {number} [defaultValue] The value used for `undefined` arguments.
+ * @returns {Function} Returns the new mathematical operation function.
+ */
+function createMathOperation(operator, defaultValue) {
+  return (value, other) => {
+    let result;
+    if (value === undefined && other === undefined) {
+      return defaultValue;
+    }
+    if (value !== undefined) {
+      result = value;
+    }
+    if (other !== undefined) {
+      if (result === undefined) {
+        return other;
+      }
+      if (typeof value == 'string' || typeof other == 'string') {
+        value = baseToString(value);
+        other = baseToString(other);
+      } else {
+        value = baseToNumber(value);
+        other = baseToNumber(other);
+      }
+      result = operator(value, other);
+    }
+    return result;
+  };
+}
+
+export default createMathOperation;