Fixed isFormData predicate; (#4413)

Added support for automatic object serialization to FormData if `Content-Type` is `multipart/form-data`;
Added support for FormData to be overloaded using `config.env.FormData` option;
Added support for FormData in node.js environment through `form-data` package;

index.d.ts

@@ -107,6 +107,9 @@ export interface AxiosRequestConfig<D = any> {
   transitional?: TransitionalOptions;
   signal?: AbortSignal;
   insecureHTTPParser?: boolean;
+  env?: {
+    FormData?: new (...args: any[]) => object;
+  };
 }
 
 export interface HeadersDefaults {

lib/adapters/http.js

@@ -87,7 +87,9 @@ module.exports = function httpAdapter(config) {
       headers['User-Agent'] = 'axios/' + VERSION;
     }
 
-    if (data && !utils.isStream(data)) {
+    if (utils.isFormData(data) && utils.isFunction(data.getHeaders)) {
+      Object.assign(headers, data.getHeaders());
+    } else if (data && !utils.isStream(data)) {
       if (Buffer.isBuffer(data)) {
         // Nothing to do...
       } else if (utils.isArrayBuffer(data)) {

lib/defaults.js

@@ -3,6 +3,7 @@
 var utils = require('./utils');
 var normalizeHeaderName = require('./helpers/normalizeHeaderName');
 var enhanceError = require('./core/enhanceError');
+var toFormData = require('./helpers/toFormData');
 
 var DEFAULT_CONTENT_TYPE = {
   'Content-Type': 'application/x-www-form-urlencoded'
@@ -71,10 +72,17 @@ var defaults = {
       setContentTypeIfUnset(headers, 'application/x-www-form-urlencoded;charset=utf-8');
       return data.toString();
     }
-    if (utils.isObject(data) || (headers && headers['Content-Type'] === 'application/json')) {
+
+    var isObjectPayload = utils.isObject(data);
+    var contentType = headers && headers['Content-Type'];
+
+    if ( isObjectPayload && contentType === 'multipart/form-data' ) {
+      return toFormData(data,  new (this.env && this.env.FormData || FormData));
+    } else if ( isObjectPayload || contentType === 'application/json' ) {
       setContentTypeIfUnset(headers, 'application/json');
       return stringifySafely(data);
     }
+
     return data;
   }],
 
@@ -112,6 +120,8 @@ var defaults = {
   maxContentLength: -1,
   maxBodyLength: -1,
 
+  env: {},
+
   validateStatus: function validateStatus(status) {
     return status >= 200 && status < 300;
   },

lib/helpers/toFormData.js

 'use strict';
 
+var utils = require('../utils');
+
 function combinedKey(parentKey, elKey) {
   return parentKey + '.' + elKey;
 }
@@ -11,7 +13,7 @@ function buildFormData(formData, data, parentKey) {
     });
   } else if (
     typeof data === 'object' &&
-    !(data instanceof File || data === null)
+    !(utils.isFile(data) || data === null)
   ) {
     Object.keys(data).forEach(function buildObject(key) {
       buildFormData(
@@ -44,10 +46,12 @@ function buildFormData(formData, data, parentKey) {
  * type FormVal = FormDataNest | FormDataPrimitive
  *
  * @param {FormVal} data
+ * @param {?Object} formData
  */
 
-module.exports = function getFormData(data) {
-  var formData = new FormData();
+module.exports = function getFormData(data, formData) {
+  // eslint-disable-next-line no-param-reassign
+  formData = formData || new FormData();
 
   buildFormData(formData, data);
 

lib/utils.js

@@ -47,15 +47,6 @@ function isArrayBuffer(val) {
   return toString.call(val) === '[object ArrayBuffer]';
 }
 
-/**
- * Determine if a value is a FormData
- *
- * @param {Object} val The value to test
- * @returns {boolean} True if value is an FormData, otherwise false
- */
-function isFormData(val) {
-  return toString.call(val) === '[object FormData]';
-}
 
 /**
  * Determine if a value is a view on an ArrayBuffer
@@ -168,6 +159,21 @@ function isStream(val) {
   return isObject(val) && isFunction(val.pipe);
 }
 
+/**
+ * Determine if a value is a FormData
+ *
+ * @param {Object} thing The value to test
+ * @returns {boolean} True if value is an FormData, otherwise false
+ */
+function isFormData(thing) {
+  var pattern = '[object FormData]';
+  return thing && (
+    (typeof FormData === 'function' && thing instanceof FormData) ||
+    toString.call(thing) === pattern ||
+    (isFunction(thing.toString) && thing.toString() === pattern)
+  );
+}
+
 /**
  * Determine if a value is a URLSearchParams object
  *

test/specs/helpers/toFormData.spec.js

 var toFormData = require("../../../lib/helpers/toFormData");
 
 describe("toFormData", function () {
-  it("Convert nested data object to FormDAta", function () {
+  it("Convert nested data object to FormData", function () {
     var o = {
       val: 123,
       nested: {