http: add new functions to OutgoingMessage

doc/api/http.md

@@ -934,6 +934,66 @@ Example:
 var contentType = response.getHeader('content-type');
 ```
 
+### response.getHeaderNames()
+<!-- YAML
+added: REPLACEME
+-->
+
+* Returns: {Array}
+
+Returns an array containing the unique names of the current outgoing headers.
+All header names are lowercase.
+
+Example:
+
+```js
+response.setHeader('Foo', 'bar');
+response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);
+
+var headerNames = response.getHeaderNames();
+// headerNames === ['foo', 'set-cookie']
+```
+
+### response.getHeaders()
+<!-- YAML
+added: REPLACEME
+-->
+
+* Returns: {Object}
+
+Returns a shallow copy of the current outgoing headers. Since a shallow copy
+is used, array values may be mutated without additional calls to various
+header-related http module methods. The keys of the returned object are the
+header names and the values are the respective header values. All header names
+are lowercase.
+
+Example:
+
+```js
+response.setHeader('Foo', 'bar');
+response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);
+
+var headers = response.getHeaders();
+// headers === { foo: 'bar', 'set-cookie': ['foo=bar', 'bar=baz'] }
+```
+
+### response.hasHeader(name)
+<!-- YAML
+added: REPLACEME
+-->
+
+* `name` {String}
+* Returns: {Boolean}
+
+Returns `true` if the header identified by `name` is currently set in the
+outgoing headers. Note that the header name matching is case-insensitive.
+
+Example:
+
+```js
+var hasContentType = response.hasHeader('content-type');
+```
+
 ### response.headersSent
 <!-- YAML
 added: v0.9.3

lib/_http_outgoing.js

@@ -18,6 +18,10 @@ var RE_FIELDS = new RegExp('^(?:Connection|Transfer-Encoding|Content-Length|' +
 var RE_CONN_VALUES = /(?:^|\W)close|upgrade(?:$|\W)/ig;
 var RE_TE_CHUNKED = common.chunkExpression;
 
+// Used to store headers returned by getHeaders()
+function OutgoingHeaders() {}
+OutgoingHeaders.prototype = Object.create(null);
+
 var dateCache;
 function utcDate() {
   if (!dateCache) {
@@ -426,6 +430,37 @@ OutgoingMessage.prototype.getHeader = function getHeader(name) {
 };
 
 
+// Returns an array of the names of the current outgoing headers.
+OutgoingMessage.prototype.getHeaderNames = function getHeaderNames() {
+  return (this._headers ? Object.keys(this._headers) : []);
+};
+
+
+// Returns a shallow copy of the current outgoing headers.
+OutgoingMessage.prototype.getHeaders = function getHeaders() {
+  const headers = this._headers;
+  const ret = new OutgoingHeaders();
+  if (headers) {
+    const keys = Object.keys(headers);
+    for (var i = 0; i < keys.length; ++i) {
+      const key = keys[i];
+      const val = headers[key][1];
+      ret[key] = val;
+    }
+  }
+  return ret;
+};
+
+
+OutgoingMessage.prototype.hasHeader = function hasHeader(name) {
+  if (typeof name !== 'string') {
+    throw new TypeError('"name" argument must be a string');
+  }
+
+  return !!(this._headers && this._headers[name.toLowerCase()]);
+};
+
+
 OutgoingMessage.prototype.removeHeader = function removeHeader(name) {
   if (typeof name !== 'string') {
     throw new TypeError('"name" argument must be a string');

test/parallel/test-http-mutable-headers.js

@@ -21,6 +21,13 @@ const cookies = [
 const s = http.createServer(common.mustCall((req, res) => {
   switch (test) {
     case 'headers':
+      // Check that header-related functions work before setting any headers
+      // eslint-disable-next-line no-restricted-properties
+      assert.deepEqual(res.getHeaders(), {});
+      assert.deepStrictEqual(res.getHeaderNames(), []);
+      assert.deepStrictEqual(res.hasHeader('Connection'), false);
+      assert.deepStrictEqual(res.getHeader('Connection'), undefined);
+
       assert.throws(() => {
         res.setHeader();
       }, /^TypeError: Header name must be a valid HTTP Token \["undefined"\]$/);
@@ -34,15 +41,52 @@ const s = http.createServer(common.mustCall((req, res) => {
         res.removeHeader();
       }, /^TypeError: "name" argument must be a string$/);
 
+      const arrayValues = [1, 2, 3];
       res.setHeader('x-test-header', 'testing');
       res.setHeader('X-TEST-HEADER2', 'testing');
       res.setHeader('set-cookie', cookies);
-      res.setHeader('x-test-array-header', [1, 2, 3]);
+      res.setHeader('x-test-array-header', arrayValues);
 
       assert.strictEqual(res.getHeader('x-test-header'), 'testing');
       assert.strictEqual(res.getHeader('x-test-header2'), 'testing');
 
+      const headersCopy = res.getHeaders();
+      // eslint-disable-next-line no-restricted-properties
+      assert.deepEqual(headersCopy, {
+        'x-test-header': 'testing',
+        'x-test-header2': 'testing',
+        'set-cookie': cookies,
+        'x-test-array-header': arrayValues
+      });
+      // eslint-disable-next-line no-restricted-properties
+      assert.deepEqual(headersCopy['set-cookie'], cookies);
+      assert.strictEqual(headersCopy['x-test-array-header'], arrayValues);
+
+      assert.deepStrictEqual(res.getHeaderNames(),
+                             ['x-test-header', 'x-test-header2',
+                              'set-cookie', 'x-test-array-header']);
+
+      assert.strictEqual(res.hasHeader('x-test-header2'), true);
+      assert.strictEqual(res.hasHeader('X-TEST-HEADER2'), true);
+      assert.strictEqual(res.hasHeader('X-Test-Header2'), true);
+      assert.throws(() => {
+        res.hasHeader();
+      }, /^TypeError: "name" argument must be a string$/);
+      assert.throws(() => {
+        res.hasHeader(null);
+      }, /^TypeError: "name" argument must be a string$/);
+      assert.throws(() => {
+        res.hasHeader(true);
+      }, /^TypeError: "name" argument must be a string$/);
+      assert.throws(() => {
+        res.hasHeader({ toString: () => 'X-TEST-HEADER2' });
+      }, /^TypeError: "name" argument must be a string$/);
+
       res.removeHeader('x-test-header2');
+
+      assert.strictEqual(res.hasHeader('x-test-header2'), false);
+      assert.strictEqual(res.hasHeader('X-TEST-HEADER2'), false);
+      assert.strictEqual(res.hasHeader('X-Test-Header2'), false);
       break;
 
     case 'contentLength':