Fixed toFormData regression bug (unreleased) with Array-like objects serialization; (#4714)

Added `toURLEncodedForm` helper;
Added automatic payload serialization to `application/x-www-form-urlencoded` to have parity with `multipart/form-data`;
Added test of handling `application/x-www-form-urlencoded` body by express.js;
Updated README.md;
Added missed param in JSDoc;
Fixed hrefs in README.md;

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

lib/helpers/buildURL.js

@@ -17,6 +17,7 @@ function encode(val) {
  *
  * @param {string} url The base of the url (e.g., http://www.google.com)
  * @param {object} [params] The params to be appended
+ * @param {?object} paramsSerializer
  * @returns {string} The formatted url
  */
 module.exports = function buildURL(url, params, paramsSerializer) {

lib/helpers/toFormData.js

@@ -20,6 +20,20 @@ function renderKey(path, key, dots) {
   }).join(dots ? '.' : '');
 }
 
+function convertValue(value) {
+  if (value === null) return '';
+
+  if (utils.isDate(value)) {
+    return value.toISOString();
+  }
+
+  if (utils.isArrayBuffer(value) || utils.isTypedArray(value)) {
+    return typeof Blob === 'function' ? new Blob([value]) : Buffer.from(value);
+  }
+
+  return value;
+}
+
 function isFlatArray(arr) {
   return utils.isArray(arr) && !arr.some(isVisitable);
 }
@@ -64,21 +78,6 @@ function toFormData(obj, formData, options) {
     throw new TypeError('visitor must be a function');
   }
 
-  function convertValue(value) {
-    if (value === null) return '';
-
-    if (utils.isDate(value)) {
-      return value.toISOString();
-    }
-
-    if (utils.isArrayBuffer(value) || utils.isTypedArray(value)) {
-      return typeof Blob === 'function' ? new Blob([value]) : Buffer.from(value);
-    }
-
-    return value;
-  }
-
-
   /**
    *
    * @param {*} value
@@ -88,7 +87,7 @@ function toFormData(obj, formData, options) {
    * @returns {boolean} return true to visit the each prop of the value recursively
    */
   function defaultVisitor(value, key, path) {
-    var arr;
+    var arr = value;
 
     if (value && !path && typeof value === 'object') {
       if (utils.endsWith(key, '{}')) {
@@ -96,7 +95,10 @@ function toFormData(obj, formData, options) {
         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)) {
+      } else if (
+        (utils.isArray(value) && isFlatArray(value)) ||
+        (utils.isFileList(value) || utils.endsWith(key, '[]') && (arr = utils.toArray(value))
+        )) {
         // eslint-disable-next-line no-param-reassign
         key = removeBrackets(key);
 
@@ -138,7 +140,7 @@ function toFormData(obj, formData, options) {
     stack.push(value);
 
     utils.forEach(value, function each(el, key) {
-      var result = !utils.isUndefined(el) && defaultVisitor.call(
+      var result = !utils.isUndefined(el) && visitor.call(
         formData, el, utils.isString(key) ? key.trim() : key, path, exposedHelpers
       );
 
@@ -150,8 +152,8 @@ function toFormData(obj, formData, options) {
     stack.pop();
   }
 
-  if (!utils.isPlainObject(obj)) {
-    throw new TypeError('data must be a plain object');
+  if (!utils.isObject(obj)) {
+    throw new TypeError('data must be an object');
   }
 
   build(obj);

lib/helpers/toURLEncodedForm.js

@@ -0,0 +1,18 @@
+'use strict';
+
+var utils = require('../utils');
+var toFormData = require('./toFormData');
+var platform = require('../platform/');
+
+module.exports = function toURLEncodedForm(data) {
+  return toFormData(data, new platform.classes.URLSearchParams(), {
+    visitor: function(value, key, path, helpers) {
+      if (platform.isNode && utils.isBuffer(value)) {
+        this.append(key, value.toString('base64'));
+        return false;
+      }
+
+      return helpers.defaultVisitor.apply(this, arguments);
+    }
+  });
+};