Array-ify repeated plain keys instead of keeping the last value

README.md

@@ -116,6 +116,23 @@ This will result in the following JSON structure:
 }
 ```
 
+### Repeated field names
+
+A field name repeated without brackets, such as a checkbox group, also becomes an array.
+
+```html
+<form>
+  <input type="checkbox" name="interests" value="music" checked />
+  <input type="checkbox" name="interests" value="sports" checked />
+</form>
+```
+
+```json
+{
+  "interests": ["music", "sports"]
+}
+```
+
 ### Complex structure
 
 You can mix dot and square bracket notation to create complex structures.

package.json

@@ -2,7 +2,7 @@
   "name": "@formspark/formson",
   "description": "FormData to JSON converter",
   "private": false,
-  "version": "0.2.1",
+  "version": "0.3.0",
   "author": "Bjorn Krols (https://formspark.io)",
   "license": "MIT",
   "keywords": [

src/index.test.ts

@@ -142,11 +142,31 @@ describe("formson", () => {
       expect(toJSON(formData)).toEqual({ note: "" });
     });
 
-    test("repeated key keeps the last value", () => {
+    test("repeated key becomes an array", () => {
       const formData = new FormData();
       formData.append("tag", "first");
       formData.append("tag", "second");
-      expect(toJSON(formData)).toEqual({ tag: "second" });
+      formData.append("tag", "third");
+      expect(toJSON(formData)).toEqual({ tag: ["first", "second", "third"] });
+    });
+
+    test("repeated key at a nested path becomes an array", () => {
+      const formData = new FormData();
+      formData.append("person.tags", "first");
+      formData.append("person.tags", "second");
+      expect(toJSON(formData)).toEqual({
+        person: { tags: ["first", "second"] },
+      });
+    });
+
+    test("repeated File values become an array", () => {
+      const a = new File(["a"], "a.txt", { type: "text/plain" });
+      const b = new File(["b"], "b.txt", { type: "text/plain" });
+      const formData = new FormData();
+      formData.append("uploads", a);
+      formData.append("uploads", b);
+      const json = toJSON(formData);
+      expect(json.uploads).toEqual([a, b]);
     });
 
     test("File values pass through unchanged", () => {
@@ -203,11 +223,11 @@ describe("formson", () => {
       expect(toJSON(formData)).toEqual({ items: { "a.b": "x" } });
     });
 
-    test("empty brackets collapse to a scalar key", () => {
+    test("empty brackets become an array, same as a repeated plain key", () => {
       const formData = new FormData();
       formData.append("items[]", "a");
       formData.append("items[]", "b");
-      expect(toJSON(formData)).toEqual({ items: "b" });
+      expect(toJSON(formData)).toEqual({ items: ["a", "b"] });
     });
 
     test("ignores prototype-pollution keys", () => {

src/index.ts

@@ -30,7 +30,20 @@ export const toJSON = function (formData: FormData): Record<string, any> {
         nextIndex <= MAX_ARRAY_INDEX;
 
       if (isLast) {
-        current[part] = value;
+        const existing = current[part];
+        if (Array.isArray(existing)) {
+          existing.push(value);
+        } else if (
+          existing !== undefined &&
+          !(typeof existing === "object" && existing.constructor === Object)
+        ) {
+          // A second plain occurrence of the same key (e.g. a checkbox
+          // group): promote to an array instead of overwriting, matching
+          // how repeated form fields are parsed server-side.
+          current[part] = [existing, value];
+        } else {
+          current[part] = value;
+        }
       } else {
         if (isNextArray) {
           if (!Array.isArray(current[part])) {