Add Response.json() static method

A method that can be used to create well-formed JSON responses with very little effort.

The JSON response is not pretty printed.

Tests: web-platform-tests/wpt#32825.

Closes #1389.

Author: lucacasonato

fetch.bs

@@ -7445,6 +7445,7 @@ interface Response {
 
   [NewObject] static Response error();
   [NewObject] static Response redirect(USVString url, optional unsigned short status = 302);
+  [NewObject] static Response json(any data, optional ResponseInit init = {});
 
   readonly attribute ResponseType type;
 
@@ -7496,6 +7497,10 @@ enum ResponseType { "basic", "cors", "default", "error", "opaque", "opaqueredire
  <dd><p>Creates a redirect {{Response}} that redirects to <var>url</var> with status
  <var>status</var>.
 
+ <dt><code><var>response</var> = <a idl>Response</a> . <a method for=Response lt=json()>json</a>(<var>data</var> [, <var>init</var>])</code>
+ <dd><p>Creates a {{Response}} whose body is the JSON-encoded <var>data</var>, and status, status
+ message, and headers are provided by <var>init</var>.
+
  <dt><code><var>response</var> . <a attribute for=Response>type</a></code>
  <dd><p>Returns <var>response</var>'s type, e.g., "<code>cors</code>".
 
@@ -7541,11 +7546,9 @@ enum ResponseType { "basic", "cors", "default", "error", "opaque", "opaqueredire
  <li><p>Return <var>responseObject</var>.
 </ol>
 
-<hr>
-
-<p>The
-<dfn constructor for=Response id=dom-response lt="Response(body, init)"><code>new Response(<var>body</var>, <var>init</var>)</code></dfn>
-constructor steps are:
+<p>To <dfn>initialize a response</dfn>, given a {{Response}} object <var>response</var>,
+{{ResponseInit}} <var>init</var>, and an optional <a for=/>body with type</a> <var>body</var>, run
+these steps:
 
 <ol>
  <li><p>If <var>init</var>["{{ResponseInit/status}}"] is not in the range 200 to 599, inclusive,
@@ -7554,50 +7557,61 @@ constructor steps are:
  <li><p>If <var>init</var>["{{ResponseInit/statusText}}"] does not match the
  <a spec=http>reason-phrase</a> token production, then <a>throw</a> a {{TypeError}}.
 
- <li><p>Set <a>this</a>'s <a for=Response>response</a> to a new <a for=/>response</a>.
-
- <li><p>Set <a>this</a>'s <a for=Response>headers</a> to a <a for=/>new</a> {{Headers}} object with
- <a>this</a>'s <a>relevant Realm</a>, whose <a for=Headers>header list</a> is <a>this</a>'s
- <a for=Response>response</a>'s <a for=response>header list</a> and <a for=Headers>guard</a> is
- "<code>response</code>".
-
- <li><p>Set <a>this</a>'s <a for=Response>response</a>'s <a for=response>status</a> to
+ <li><p>Set <var>response</var>'s <a for=Response>response</a>'s <a for=response>status</a> to
  <var>init</var>["{{ResponseInit/status}}"].
 
- <li><p>Set <a>this</a>'s <a for=Response>response</a>'s <a for=response>status message</a> to
- <var>init</var>["{{ResponseInit/statusText}}"].
+ <li><p>Set <var>response</var>'s <a for=Response>response</a>'s <a for=response>status message</a>
+ to <var>init</var>["{{ResponseInit/statusText}}"].
 
  <li><p>If <var>init</var>["{{ResponseInit/headers}}"] <a for=map>exists</a>, then
- <a for=Headers>fill</a> <a>this</a>'s <a for=Response>headers</a> with
+ <a for=Headers>fill</a> <var>response</var>'s <a for=Response>headers</a> with
  <var>init</var>["{{ResponseInit/headers}}"].
 
  <li>
-  <p>If <var>body</var> is non-null, then:
+  <p>If <var>body</var> was given, then:
 
   <ol>
    <li>
-    <p>If <var>init</var>["{{ResponseInit/status}}"] is a <a>null body status</a>, then <a>throw</a>
-    a {{TypeError}}.
+    <p>If <var>response</var>'s <a for=response>status</a> is a <a>null body status</a>, then
+    <a>throw</a> a {{TypeError}}.
 
     <p class="note no-backref">101 is included in <a>null body status</a> due to its use elsewhere.
     It does not affect this step.
 
-   <li><p>Let <var>bodyWithType</var> be the result of <a for=BodyInit>extracting</a>
-   <var>body</var>.
-
-   <li><p>Set <a>this</a>'s <a for=Response>response</a>'s <a for=response>body</a> to
-   <var>bodyWithType</var>'s <a for="body with type">body</a>.
+   <li><p>Set <var>response</var>'s <a for=response>body</a> to <var>body</var>'s
+   <a for="body with type">body</a>.
 
-   <li><p>Let <var>type</var> be <var>bodyWithType</var>'s <a for="body with type">type</a>.
-
-   <li><p>If <var>type</var> is non-null and <a>this</a>'s <a for=Response>response</a>'s
-   <a for=response>header list</a> <a for="header list">does not contain</a>
+   <li><p>If <var>body</var>'s <a for="body with type">type</a> is non-null and
+   <var>response</var>'s <a for=response>header list</a> <a for="header list">does not contain</a>
    `<code>Content-Type</code>`, then <a for="header list">append</a> (`<code>Content-Type</code>`,
-   <var>type</var>) to <a>this</a>'s <a for=Response>response</a>'s
+   <var>body</var>'s <a for="body with type">type</a>) to <var>response</var>'s
    <a for=response>header list</a>.
   </ol>
 </ol>
 
+<hr>
+
+<p>The
+<dfn constructor for=Response id=dom-response lt="Response(body, init)"><code>new Response(<var>body</var>, <var>init</var>)</code></dfn>
+constructor steps are:
+
+<ol>
+ <li><p>Set <a>this</a>'s <a for=Response>response</a> to a new <a for=/>response</a>.
+
+ <li><p>Set <a>this</a>'s <a for=Response>headers</a> to a <a for=/>new</a> {{Headers}} object with
+ <a>this</a>'s <a>relevant Realm</a>, whose <a for=Headers>header list</a> is <a>this</a>'s
+ <a for=Response>response</a>'s <a for=response>header list</a> and <a for=Headers>guard</a> is
+ "<code>response</code>".
+
+ <li><p>Let <var>bodyWithType</var> be null.
+
+ <li><p>If <var>body</var> is non-null, then set <var>bodyWithType</var> to the result of
+ <a for=BodyInit>extracting</a> <var>body</var>.
+
+ <li><p>Perform <a>initialize a response</a> given <a>this</a>, <var>init</var>, and
+ <var>bodyWithType</var>.
+</ol>
+
 <p>The static <dfn method for=Response><code>error()</code></dfn> method steps are to return the
 result of <a for=Response>creating</a> a {{Response}} object, given a new <a>network error</a>,
 "<code>immutable</code>", and <a>this</a>'s <a>relevant Realm</a>.
@@ -7630,6 +7644,27 @@ are:
  <li><p>Return <var>responseObject</var>.
 </ol>
 
+
+<p>The static
+<dfn method for=Response><code>json(<var>data</var>, <var>init</var>)</code></dfn> method steps
+are:
+
+<ol>
+ <li><p>Let <var>bytes</var> the result of running <a>serialize a JavaScript value to JSON bytes</a>
+ on <var>data</var>.
+
+ <li><p>Let <var>body</var> be the result of <a for=BodyInit>extracting</a> <var>bytes</var>.
+
+ <li><p>Let <var>responseObject</var> be the result of <a for=Response>creating</a> a {{Response}}
+ object, given a new <a for=/>response</a>, "<code>response</code>", and <a>this</a>'s
+ <a>relevant Realm</a>.
+
+ <li><p>Perform <a>initialize a response</a> given <var>responseObject</var>, <var>init</var>, and
+ (<var>body</var>, "<code>application/json</code>").
+
+ <li><p>Return <var>responseObject</var>.
+</ol>
+
 <p>The <dfn attribute for=Response><code>type</code></dfn> getter steps are to return <a>this</a>'s
 <a for=Response>response</a>'s <a for=response>type</a>.