feat: implement response.has (#1397)

docs/api/response.md

@@ -177,6 +177,15 @@ app.use(async ctx => {
 const etag = ctx.response.get('ETag');
 ```
 
+### response.has(field)
+
+  Returns `true` if the header identified by name is currently set in the outgoing headers.
+  The header name matching is case-insensitive.
+
+```js
+const rateLimited = ctx.response.has('X-RateLimit-Limit');
+```
+
 ### response.set(field, value)
 
   Set response header `field` to `value`:

lib/context.js

@@ -193,6 +193,7 @@ delegate(proto, 'response')
   .method('redirect')
   .method('remove')
   .method('vary')
+  .method('has')
   .method('set')
   .method('append')
   .method('flushHeaders')

lib/response.js

@@ -149,7 +149,7 @@ module.exports = {
     if (!this._explicitStatus) this.status = 200;
 
     // set the content-type only if not yet set
-    const setType = !this.header['content-type'];
+    const setType = !this.has('Content-Type');
 
     // string
     if ('string' == typeof val) {
@@ -419,6 +419,29 @@ module.exports = {
     return this.header[field.toLowerCase()] || '';
   },
 
+  /**
+   * Returns true if the header identified by name is currently set in the outgoing headers.
+   * The header name matching is case-insensitive.
+   *
+   * Examples:
+   *
+   *     this.has('Content-Type');
+   *     // => true
+   *
+   *     this.get('content-type');
+   *     // => true
+   *
+   * @param {String} field
+   * @return {boolean}
+   * @api public
+   */
+  has(field) {
+    return typeof this.res.hasHeader === 'function'
+      ? this.res.hasHeader(field)
+      // Node < 7.7
+      : field.toLowerCase() in this.headers;
+  },
+
   /**
    * Set header `field` to `val`, or pass
    * an object of header fields.

test/response/has.js

@@ -0,0 +1,21 @@
+
+'use strict';
+
+const assert = require('assert');
+const context = require('../helpers/context');
+
+describe('ctx.response.has(name)', () => {
+  it('should check a field value, case insensitive way', () => {
+    const ctx = context();
+    ctx.set('X-Foo', '');
+    assert.ok(ctx.response.has('x-Foo'));
+    assert.ok(ctx.has('x-foo'));
+  });
+
+  it('should return false for non-existent header', () => {
+    const ctx = context();
+    assert.strictEqual(ctx.response.has('boo'), false);
+    ctx.set('x-foo', 5);
+    assert.strictEqual(ctx.has('x-boo'), false);
+  });
+});