test: add Node unit tests for toFormData and refactor buildURL to avoid param reassignment (#7272)

Co-authored-by: Jay <jasonsaayman@gmail.com>

lib/helpers/buildURL.js

@@ -29,29 +29,26 @@ function encode(val) {
  * @returns {string} The formatted url
  */
 export default function buildURL(url, params, options) {
-  /*eslint no-param-reassign:0*/
   if (!params) {
     return url;
   }
 
   const _encode = options && options.encode || encode;
 
-  if (utils.isFunction(options)) {
+  const _options = utils.isFunction(options) ? {
-    options = {
+    serialize: options
-      serialize: options
+  } : options;
-    };
-  } 
 
-  const serializeFn = options && options.serialize;
+  const serializeFn = _options && _options.serialize;
 
   let serializedParams;
 
   if (serializeFn) {
-    serializedParams = serializeFn(params, options);
+    serializedParams = serializeFn(params, _options);
   } else {
     serializedParams = utils.isURLSearchParams(params) ?
       params.toString() :
-      new AxiosURLSearchParams(params, options).toString(_encode);
+      new AxiosURLSearchParams(params, _options).toString(_encode);
   }
 
   if (serializedParams) {

test/unit/helpers/toFormData.js

@@ -0,0 +1,53 @@
+import assert from 'assert';
+import toFormData from '../../../lib/helpers/toFormData.js';
+import FormData from 'form-data';
+
+describe('helpers::toFormData', function () {
+  it('should convert a flat object to FormData', function () {
+    const data = {
+      foo: 'bar',
+      baz: 123
+    };
+
+    const formData = toFormData(data, new FormData());
+    
+    assert.ok(formData instanceof FormData);
+    // form-data package specific checks
+    assert.ok(formData._streams.length > 0);
+  });
+
+  it('should convert a nested object to FormData', function () {
+    const data = {
+      foo: {
+        bar: 'baz'
+      }
+    };
+
+    const formData = toFormData(data, new FormData());
+    
+    assert.ok(formData instanceof FormData);
+  });
+
+  it('should throw Error on circular reference', function () {
+    const data = {
+      foo: 'bar'
+    };
+    data.self = data;
+
+    try {
+      toFormData(data, new FormData());
+      assert.fail('Should have thrown an error');
+    } catch (e) {
+      assert.strictEqual(e.message, 'Circular reference detected in self');
+    }
+  });
+
+  it('should handle arrays', function () {
+    const data = {
+      arr: [1, 2, 3]
+    };
+
+    const formData = toFormData(data, new FormData());
+    assert.ok(formData instanceof FormData);
+  });
+});