fix(params): re-added the ability to set the function as paramsSerializer config; (#5633)

index.d.cts

@@ -370,7 +370,7 @@ declare namespace axios {
     transformResponse?: AxiosResponseTransformer | AxiosResponseTransformer[];
     headers?: (RawAxiosRequestHeaders & MethodsHeaders) | AxiosHeaders;
     params?: any;
-    paramsSerializer?: ParamsSerializerOptions;
+    paramsSerializer?: ParamsSerializerOptions | CustomParamsSerializer;
     data?: D;
     timeout?: Milliseconds;
     timeoutErrorMessage?: string;

index.d.ts

@@ -311,7 +311,7 @@ export interface AxiosRequestConfig<D = any> {
   transformResponse?: AxiosResponseTransformer | AxiosResponseTransformer[];
   headers?: (RawAxiosRequestHeaders & MethodsHeaders) | AxiosHeaders;
   params?: any;
-  paramsSerializer?: ParamsSerializerOptions;
+  paramsSerializer?: ParamsSerializerOptions | CustomParamsSerializer;
   data?: D;
   timeout?: Milliseconds;
   timeoutErrorMessage?: string;

lib/core/Axios.js

@@ -57,11 +57,17 @@ class Axios {
       }, false);
     }
 
-    if (paramsSerializer !== undefined) {
-      validator.assertOptions(paramsSerializer, {
-        encode: validators.function,
-        serialize: validators.function
-      }, true);
+    if (paramsSerializer != null) {
+      if (utils.isFunction(paramsSerializer)) {
+        config.paramsSerializer = {
+          serialize: paramsSerializer
+        }
+      } else {
+        validator.assertOptions(paramsSerializer, {
+          encode: validators.function,
+          serialize: validators.function
+        }, true);
+      }
     }
 
     // Set config.method

test/unit/adapters/http.js

@@ -2103,4 +2103,18 @@ describe('supports http with nodejs', function () {
   it('should properly handle synchronous errors inside the adapter', function () {
     return assert.rejects(() => axios.get('http://192.168.0.285'), /Invalid URL/);
   });
+
+  it('should support function as paramsSerializer value', async () => {
+    server = await startHTTPServer((req, res) => res.end(req.url));
+
+    const {data} = await axios.post(LOCAL_SERVER_URL, 'test', {
+      params: {
+        x: 1
+      },
+      paramsSerializer: () => 'foo',
+      maxRedirects: 0
+    });
+
+    assert.strictEqual(data, '/?foo');
+  });
 });