fix(adapters): improved adapters loading logic to have clear error messages;

lib/adapters/adapters.js

@@ -9,7 +9,7 @@ const knownAdapters = {
 }
 
 utils.forEach(knownAdapters, (fn, value) => {
-  if(fn) {
+  if (fn) {
     try {
       Object.defineProperty(fn, 'name', {value});
     } catch (e) {
@@ -19,6 +19,10 @@ utils.forEach(knownAdapters, (fn, value) => {
   }
 });
 
+const renderReason = (reason) => `- ${reason}`;
+
+const isResolvedHandle = (adapter) => utils.isFunction(adapter) || adapter === null || adapter === false;
+
 export default {
   getAdapter: (adapters) => {
     adapters = utils.isArray(adapters) ? adapters : [adapters];
@@ -27,30 +31,44 @@ export default {
     let nameOrAdapter;
     let adapter;
 
+    const rejectedReasons = {};
+
     for (let i = 0; i < length; i++) {
       nameOrAdapter = adapters[i];
-      if((adapter = utils.isString(nameOrAdapter) ? knownAdapters[nameOrAdapter.toLowerCase()] : nameOrAdapter)) {
+      let id;
+
+      adapter = nameOrAdapter;
+
+      if (!isResolvedHandle(nameOrAdapter)) {
+        adapter = knownAdapters[(id = String(nameOrAdapter)).toLowerCase()];
+
+        if (adapter === undefined) {
+          throw new AxiosError(`Unknown adapter '${id}'`);
+        }
+      }
+
+      if (adapter) {
         break;
       }
+
+      rejectedReasons[id || '#' + i] = adapter;
     }
 
     if (!adapter) {
-      if (adapter === false) {
-        throw new AxiosError(
-          `Adapter ${nameOrAdapter} is not supported by the environment`,
-          'ERR_NOT_SUPPORT'
+
+      const reasons = Object.entries(rejectedReasons)
+        .map(([id, state]) => `adapter ${id} ` +
+          (state === false ? 'is not supported by the environment' : 'is not available in the build')
         );
-      }
 
-      throw new Error(
-        utils.hasOwnProp(knownAdapters, nameOrAdapter) ?
-          `Adapter '${nameOrAdapter}' is not available in the build` :
-          `Unknown adapter '${nameOrAdapter}'`
-      );
-    }
+      let s = length ?
+        (reasons.length > 1 ? 'since :\n' + reasons.map(renderReason).join('\n') : ' ' + renderReason(reasons[0])) :
+        'as no adapter specified';
 
-    if (!utils.isFunction(adapter)) {
-      throw new TypeError('adapter is not a function');
+      throw new AxiosError(
+        `There is no suitable adapter to dispatch the request ` + s,
+        'ERR_NOT_SUPPORT'
+      );
     }
 
     return adapter;

lib/defaults/index.js

@@ -37,7 +37,7 @@ const defaults = {
 
   transitional: transitionalDefaults,
 
-  adapter: platform.isNode ? 'http' : 'xhr',
+  adapter: ['xhr', 'http'],
 
   transformRequest: [function transformRequest(data, headers) {
     const contentType = headers.getContentType() || '';

test/unit/adapters/adapters.js

@@ -0,0 +1,48 @@
+import adapters from '../../../lib/adapters/adapters.js';
+import assert from 'assert';
+
+
+describe('adapters', function () {
+  const store = {...adapters.adapters};
+
+  beforeEach(() => {
+    Object.keys(adapters.adapters).forEach((name) => {
+      delete adapters.adapters[name];
+    });
+
+    Object.assign(adapters.adapters, store);
+  });
+
+  it('should support loading by fn handle', function () {
+    const adapter = () => {};
+    assert.strictEqual(adapters.getAdapter(adapter), adapter);
+  });
+
+  it('should support loading by name', function () {
+    const adapter = () => {};
+    adapters.adapters['testadapter'] = adapter;
+    assert.strictEqual(adapters.getAdapter('testAdapter'), adapter);
+  });
+
+  it('should detect adapter unavailable status', function () {
+    adapters.adapters['testadapter'] = null;
+    assert.throws(()=> adapters.getAdapter('testAdapter'), /is not available in the build/)
+  });
+
+  it('should detect adapter unsupported status', function () {
+    adapters.adapters['testadapter'] = false;
+    assert.throws(()=> adapters.getAdapter('testAdapter'), /is not supported by the environment/)
+  });
+
+  it('should pick suitable adapter from the list', function () {
+    const adapter = () => {};
+
+    Object.assign(adapters.adapters, {
+      foo: false,
+      bar: null,
+      baz: adapter
+    });
+
+    assert.strictEqual(adapters.getAdapter(['foo', 'bar', 'baz']), adapter);
+  });
+});