JSON improvements: throw if JSON parsing failed; number, boolean can be passed directly as payload for encoding to JSON #2613, #61, #907 (#3688)

* Draft

* Added support for primitive types to be converted to JSON if the request Content-Type is 'application/json';
Added throwing SyntaxError if JSON parsing failed and responseType is json;
Added transitional option object;
Added options validator to assert transitional options;
Added transitional option `silentJSONParsing= true` for backward compatibility;
Updated README.md;
Updated typings;

* Fixed isOlderVersion helper;
Fixed typo;
Added validator.spec.js;

* Added forcedJSONParsing transitional option #2791

* `transformData` is now called in the default configuration context if the function context is not specified (for tests compatibility);

* Added `transitional.clarifyTimeoutError` to throw ETIMEDOUT error instead of generic ECONNABORTED on request timeouts;
Added support of onloadend handler if available instead of onreadystatechange;
Added xhr timeout test;
Fixed potential bug of xhr adapter with proper handling timeouts&errors (FakeXMLHTTPRequest failed to handle timeouts);

lib/helpers/validator.js

@@ -0,0 +1,105 @@
+'use strict';
+
+var pkg = require('./../../package.json');
+
+var validators = {};
+
+// eslint-disable-next-line func-names
+['object', 'boolean', 'number', 'function', 'string', 'symbol'].forEach(function(type, i) {
+  validators[type] = function validator(thing) {
+    return typeof thing === type || 'a' + (i < 1 ? 'n ' : ' ') + type;
+  };
+});
+
+var deprecatedWarnings = {};
+var currentVerArr = pkg.version.split('.');
+
+/**
+ * Compare package versions
+ * @param {string} version
+ * @param {string?} thanVersion
+ * @returns {boolean}
+ */
+function isOlderVersion(version, thanVersion) {
+  var pkgVersionArr = thanVersion ? thanVersion.split('.') : currentVerArr;
+  var destVer = version.split('.');
+  for (var i = 0; i < 3; i++) {
+    if (pkgVersionArr[i] > destVer[i]) {
+      return true;
+    } else if (pkgVersionArr[i] < destVer[i]) {
+      return false;
+    }
+  }
+  return false;
+}
+
+/**
+ * Transitional option validator
+ * @param {function|boolean?} validator
+ * @param {string?} version
+ * @param {string} message
+ * @returns {function}
+ */
+validators.transitional = function transitional(validator, version, message) {
+  var isDeprecated = version && isOlderVersion(version);
+
+  function formatMessage(opt, desc) {
+    return '[Axios v' + pkg.version + '] Transitional option \'' + opt + '\'' + desc + (message ? '. ' + message : '');
+  }
+
+  // eslint-disable-next-line func-names
+  return function(value, opt, opts) {
+    if (validator === false) {
+      throw new Error(formatMessage(opt, ' has been removed in ' + version));
+    }
+
+    if (isDeprecated && !deprecatedWarnings[opt]) {
+      deprecatedWarnings[opt] = true;
+      // eslint-disable-next-line no-console
+      console.warn(
+        formatMessage(
+          opt,
+          ' has been deprecated since v' + version + ' and will be removed in the near future'
+        )
+      );
+    }
+
+    return validator ? validator(value, opt, opts) : true;
+  };
+};
+
+/**
+ * Assert object's properties type
+ * @param {object} options
+ * @param {object} schema
+ * @param {boolean?} allowUnknown
+ */
+
+function assertOptions(options, schema, allowUnknown) {
+  if (typeof options !== 'object') {
+    throw new TypeError('options must be an object');
+  }
+  var keys = Object.keys(options);
+  var i = keys.length;
+  while (i-- > 0) {
+    var opt = keys[i];
+    var validator = schema[opt];
+    if (validator) {
+      var value = options[opt];
+      var result = value === undefined || validator(value, opt, options);
+      if (result !== true) {
+        throw new TypeError('option ' + opt + ' must be ' + result);
+      }
+      continue;
+    }
+    if (allowUnknown !== true) {
+      throw Error('Unknown option ' + opt);
+    }
+  }
+}
+
+module.exports = {
+  isOlderVersion: isOlderVersion,
+  assertOptions: assertOptions,
+  validators: validators
+};