Added enhanced toFormData implementation with additional options

README.md

@@ -901,7 +901,56 @@ axios.post('https://httpbin.org/post', {x: 1, buf: new Buffer(10)}, {
 Axios FormData serializer supports some special endings to perform the following operations:
 
 - `{}` - serialize the value with JSON.stringify
-- `[]` - unwrap the array like object as separate fields with the same key 
+- `[]` - unwrap the array-like object as separate fields with the same key 
+
+> NOTE: 
+> unwrap/expand operation will be used by default on array-like objects
+
+FormData serializer supports additional options via `config.formSerializer: object` property to handle rare cases:
+
+- `visitor: Function` - user-defined visitor function that will be called recursively to serialize the data object
+to a `FormData` object by following custom rules.
+
+- `dots: boolean = false` - use dot notation instead of brackets to serialize arrays and objects;
+
+- `metaTokens: boolean = true` - add the special ending (e.g `user{}: '{"name": "John"}'`) in the FormData key. 
+The back-end body-parser could potentially use this meta-information to automatically parse the value as JSON.
+
+- `indexes: null|false|true = false` - controls how indexes will be added to unwrapped keys of `flat` array-like objects
+
+    - `null` - don't add brackets (`arr: 1`, `arr: 2`, `arr: 3`) 
+    - `false`(default) - add empty brackets (`arr[]: 1`, `arr[]: 2`, `arr[]: 3`)
+    - `true` - add brackets with indexes  (`arr[0]: 1`, `arr[1]: 2`, `arr[2]: 3`)
+
+Let's say we have an object like this one:
+
+```js
+const obj = {
+  x: 1,
+  arr: [1, 2, 3],
+  arr2: [1, [2], 3],
+  users: [{name: 'Peter', surname: 'Griffin'}, {name: 'Thomas', surname: 'Anderson'}],
+  'obj2{}': [{x:1}]
+};
+```
+
+The following steps will be executed by the Axios serializer internally:
+
+```js
+const formData= new FormData();
+formData.append('x', '1');
+formData.append('arr[]', '1');
+formData.append('arr[]', '2');
+formData.append('arr[]', '3');
+formData.append('arr2[0]', '1');
+formData.append('arr2[1][0]', '2');
+formData.append('arr2[2]', '3');
+formData.append('users[0][name]', 'Peter');
+formData.append('users[0][surname]', 'Griffin');
+formData.append('users[1][name]', 'Thomas');
+formData.append('users[1][surname]', 'Anderson');
+formData.append('obj2{}', '[{"x":1}]');
+```
 
 ```js
 const axios= require('axios');
@@ -917,9 +966,9 @@ axios.post('https://httpbin.org/post', {
 ```
 
 Axios supports the following shortcut methods: `postForm`, `putForm`, `patchForm`
-which are just the corresponding http methods with a header preset: `Content-Type`: `multipart/form-data`.
+which are just the corresponding http methods with the content-type header preset to `multipart/form-data`.
 
-FileList object can be passed directly:
+`FileList` object can be passed directly:
 
 ```js
 await axios.postForm('https://httpbin.org/post', document.querySelector('#fileInput').files)

index.d.ts

@@ -79,6 +79,23 @@ export interface GenericAbortSignal {
   removeEventListener: (...args: any) => any;
 }
 
+export interface FormDataVisitorHelpers {
+  defaultVisitor: FormDataVisitor;
+  convertValue: (value: any) => any;
+  isVisitable: (value: any) => boolean;
+}
+
+export interface FormDataVisitor {
+  (value: any, key: string | number, path: null | Array<string | number>, helpers: FormDataVisitorHelpers): boolean;
+}
+
+export interface FormSerializerOptions {
+  visitor?: FormDataVisitor;
+  dots?: boolean;
+  metaTokens?: boolean;
+  indexes?: boolean;
+}
+
 export interface AxiosRequestConfig<D = any> {
   url?: string;
   method?: Method | string;
@@ -117,6 +134,7 @@ export interface AxiosRequestConfig<D = any> {
   env?: {
     FormData?: new (...args: any[]) => object;
   };
+  formSerializer?: FormSerializerOptions;
 }
 
 export interface HeadersDefaults {
@@ -268,7 +286,7 @@ export interface AxiosStatic extends AxiosInstance {
   all<T>(values: Array<T | Promise<T>>): Promise<T[]>;
   spread<T, R>(callback: (...args: T[]) => R): (array: T[]) => R;
   isAxiosError(payload: any): payload is AxiosError;
-  toFormData(sourceObj: object, targetFormData?: GenericFormData): GenericFormData;
+  toFormData(sourceObj: object, targetFormData?: GenericFormData, options?: FormSerializerOptions): GenericFormData;
 }
 
 declare const axios: AxiosStatic;

lib/defaults/index.js

@@ -77,7 +77,11 @@ var defaults = {
 
     if ((isFileList = utils.isFileList(data)) || (isObjectPayload && contentType === 'multipart/form-data')) {
       var _FormData = this.env && this.env.FormData;
-      return toFormData(isFileList ? {'files[]': data} : data, _FormData && new _FormData());
+      return toFormData(
+        isFileList ? {'files[]': data} : data,
+        _FormData && new _FormData(),
+        this.formSerializer
+      );
     } else if (isObjectPayload || contentType === 'application/json') {
       setContentTypeIfUnset(headers, 'application/json');
       return stringifySafely(data);

lib/helpers/toFormData.js

@@ -1,19 +1,68 @@
 'use strict';
 
 var utils = require('../utils');
+var envFormData = require('../defaults/env/FormData');
+
+function isVisitable(thing) {
+  return utils.isPlainObject(thing) || utils.isArray(thing);
+}
+
+function removeBrackets(key) {
+  return utils.endsWith(key, '[]') ? key.slice(0, -2) : key;
+}
+
+function renderKey(path, key, dots) {
+  if (!path) return key;
+  return path.concat(key).map(function each(token, i) {
+    // eslint-disable-next-line no-param-reassign
+    token = removeBrackets(token);
+    return !dots && i ? '[' + token + ']' : token;
+  }).join(dots ? '.' : '');
+}
+
+function isFlatArray(arr) {
+  return utils.isArray(arr) && !arr.some(isVisitable);
+}
+
+var predicates = utils.toFlatObject(utils, {}, null, function filter(prop) {
+  return /^is[A-Z]/.test(prop);
+});
 
 /**
  * Convert a data object to FormData
  * @param {Object} obj
  * @param {?Object} [formData]
+ * @param {?Object} [options]
+ * @param {Function} [options.visitor]
+ * @param {Boolean} [options.metaTokens = true]
+ * @param {Boolean} [options.dots = false]
+ * @param {?Boolean} [options.indexes = false]
  * @returns {Object}
  **/
 
-function toFormData(obj, formData) {
+function toFormData(obj, formData, options) {
   // eslint-disable-next-line no-param-reassign
-  formData = formData || new FormData();
+  formData = formData || new (envFormData || FormData)();
 
-  var stack = [];
+  // eslint-disable-next-line no-param-reassign
+  options = utils.toFlatObject(options, {
+    metaTokens: true,
+    dots: false,
+    indexes: false
+  }, false, function defined(option, source) {
+    // eslint-disable-next-line no-eq-null,eqeqeq
+    return !utils.isUndefined(source[option]);
+  });
+
+  var metaTokens = options.metaTokens;
+  // eslint-disable-next-line no-use-before-define
+  var visitor = options.visitor || defaultVisitor;
+  var dots = options.dots;
+  var indexes = options.indexes;
+
+  if (!utils.isFunction(visitor)) {
+    throw new TypeError('visitor must be a function');
+  }
 
   function convertValue(value) {
     if (value === null) return '';
@@ -29,39 +78,80 @@ function toFormData(obj, formData) {
     return value;
   }
 
-  function build(data, parentKey) {
-    if (utils.isPlainObject(data) || utils.isArray(data)) {
-      if (stack.indexOf(data) !== -1) {
-        throw Error('Circular reference detected in ' + parentKey);
+
+  /**
+   *
+   * @param {*} value
+   * @param {String|Number} key
+   * @param {Array<String|Number>} path
+   * @this {FormData}
+   * @returns {boolean} return true to visit the each prop of the value recursively
+   */
+  function defaultVisitor(value, key, path) {
+    var arr;
+
+    if (value && !path && typeof value === 'object') {
+      if (utils.endsWith(key, '{}')) {
+        // eslint-disable-next-line no-param-reassign
+        key = metaTokens ? key : key.slice(0, -2);
+        // eslint-disable-next-line no-param-reassign
+        value = JSON.stringify(value);
+      } else if (!utils.isPlainObject(value) && (arr = utils.toArray(value)) && isFlatArray(arr)) {
+        // eslint-disable-next-line no-param-reassign
+        key = removeBrackets(key);
+
+        arr.forEach(function each(el, index) {
+          !utils.isUndefined(el) && formData.append(
+            // eslint-disable-next-line no-nested-ternary
+            indexes === true ? renderKey([key], index, dots) : (indexes === null ? key : key + '[]'),
+            convertValue(el)
+          );
+        });
+        return false;
       }
+    }
+
+    if (isVisitable(value)) {
+      return true;
+    }
+
+    formData.append(renderKey(path, key, dots), convertValue(value));
+
+    return false;
+  }
+
+  var stack = [];
+
+  var exposedHelpers = Object.assign(predicates, {
+    defaultVisitor: defaultVisitor,
+    convertValue: convertValue,
+    isVisitable: isVisitable
+  });
+
+  function build(value, path) {
+    if (utils.isUndefined(value)) return;
 
-      stack.push(data);
-
-      utils.forEach(data, function each(value, key) {
-        if (utils.isUndefined(value)) return;
-        var fullKey = parentKey ? parentKey + '.' + key : key;
-        var arr;
-
-        if (value && !parentKey && typeof value === 'object') {
-          if (utils.endsWith(key, '{}')) {
-            // eslint-disable-next-line no-param-reassign
-            value = JSON.stringify(value);
-          } else if (utils.endsWith(key, '[]') && (arr = utils.toArray(value))) {
-            // eslint-disable-next-line func-names
-            arr.forEach(function(el) {
-              !utils.isUndefined(el) && formData.append(fullKey, convertValue(el));
-            });
-            return;
-          }
-        }
-
-        build(value, fullKey);
-      });
-
-      stack.pop();
-    } else {
-      formData.append(parentKey, convertValue(data));
+    if (stack.indexOf(value) !== -1) {
+      throw Error('Circular reference detected in ' + path.join('.'));
     }
+
+    stack.push(value);
+
+    utils.forEach(value, function each(el, key) {
+      var result = !utils.isUndefined(el) && defaultVisitor.call(
+        formData, el, utils.isString(key) ? key.trim() : key, path, exposedHelpers
+      );
+
+      if (result === true) {
+        build(el, path ? path.concat(key) : [key]);
+      }
+    });
+
+    stack.pop();
+  }
+
+  if (!utils.isPlainObject(obj)) {
+    throw new TypeError('data must be a plain object');
   }
 
   build(obj);

lib/utils.js

@@ -366,29 +366,32 @@ function inherits(constructor, superConstructor, props, descriptors) {
  * Resolve object with deep prototype chain to a flat object
  * @param {Object} sourceObj source object
  * @param {Object} [destObj]
- * @param {Function} [filter]
+ * @param {Function|Boolean} [filter]
+ * @param {Function} [propFilter]
  * @returns {Object}
  */
 
-function toFlatObject(sourceObj, destObj, filter) {
+function toFlatObject(sourceObj, destObj, filter, propFilter) {
   var props;
   var i;
   var prop;
   var merged = {};
 
   destObj = destObj || {};
+  // eslint-disable-next-line no-eq-null,eqeqeq
+  if (sourceObj == null) return destObj;
 
   do {
     props = Object.getOwnPropertyNames(sourceObj);
     i = props.length;
     while (i-- > 0) {
       prop = props[i];
-      if (!merged[prop]) {
+      if ((!propFilter || propFilter(prop, sourceObj, destObj)) && !merged[prop]) {
         destObj[prop] = sourceObj[prop];
         merged[prop] = true;
       }
     }
-    sourceObj = Object.getPrototypeOf(sourceObj);
+    sourceObj = filter !== false && Object.getPrototypeOf(sourceObj);
   } while (sourceObj && (!filter || filter(sourceObj, destObj)) && sourceObj !== Object.prototype);
 
   return destObj;
@@ -413,14 +416,15 @@ function endsWith(str, searchString, position) {
 
 
 /**
- * Returns new array from array like object
+ * Returns new array from array like object or null if failed
  * @param {*} [thing]
- * @returns {Array}
+ * @returns {?Array}
  */
 function toArray(thing) {
   if (!thing) return null;
+  if (isArray(thing)) return thing;
   var i = thing.length;
-  if (isUndefined(i)) return null;
+  if (!isNumber(i)) return null;
   var arr = new Array(i);
   while (i-- > 0) {
     arr[i] = thing[i];