Testing and cleanup of transformResponse (axios#3377)

- Tests for transformResponse
 - Remove eslint error by renaming the var
 - Test that there data a length to avoid JSON.parse headache
 - Use `util.isString()` over `typeof`

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

lib/defaults.js

@@ -55,13 +55,13 @@ var defaults = {
   }],
 
   transformResponse: [function transformResponse(data) {
-    /*eslint no-param-reassign:0*/
-    if (typeof data === 'string') {
+    var result = data;
+    if (utils.isString(result) && result.length) {
       try {
-        data = JSON.parse(data);
+        result = JSON.parse(result);
       } catch (e) { /* Ignore */ }
     }
-    return data;
+    return result;
   }],
 
   /**

test/unit/defaults/transformReponse.js

@@ -0,0 +1,30 @@
+var defaults = require('../../../lib/defaults');
+var transformData = require('../../../lib/core/transformData');
+var assert = require('assert');
+
+describe('transformResponse', function () {
+    describe('200 request', function () {
+        it('parses json', function () {
+            var data = '{"message": "hello, world"}';
+            var result = transformData(data, {'content-type': 'application/json'}, defaults.transformResponse);
+            assert.strictEqual(result.message, 'hello, world');
+        });
+        it('ignores XML', function () {
+            var data = '<message>hello, world</message>';
+            var result = transformData(data, {'content-type': 'text/xml'}, defaults.transformResponse);
+            assert.strictEqual(result, data);
+        });
+    });
+    describe('204 request', function () {
+        it('does not parse the empty string', function () {
+            var data = '';
+            var result = transformData(data, {'content-type': undefined}, defaults.transformResponse);
+            assert.strictEqual(result, '');
+        });
+        it('does not parse undefined', function () {
+            var data = undefined;
+            var result = transformData(data, {'content-type': undefined}, defaults.transformResponse);
+            assert.strictEqual(result, data);
+        });
+    });
+});