Re-add JSON module scripts

Reland JSON modules (#4315), originally found in db03474 but removed in a530f6f. Importing a JSON module now requires the module type to be specified at every import site with an import assertion, addressing the security concern that led to the revert.

Additionally, some of the infrastructure now lives in the TC39 proposal at https://github.com/tc39/proposal-json-modules, and 3d45584 introduced the import assertions integration, so overall this patch is pretty small.

Author: dandclark

source

@@ -2888,13 +2888,15 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li>The <dfn data-x="js-HostGetSupportedImportAssertions" data-x-href="https://tc39.es/proposal-import-assertions/#sec-hostgetsupportedimportassertions">HostGetSupportedImportAssertions</dfn> abstract operation</li>
     </ul>
 
-    <p>The following terms are defined in the <cite>JSON modules</cite> proposal and used in this
-    specification: <ref spec=JSJSONMODULES></p>
+    <p>User agents that support JavaScript must also implement the <cite>JSON modules</cite>
+    proposal. The following terms are defined there, and used in this specification: <ref
+    spec=JSJSONMODULES></p>
 
     <ul class="brief">
-     <li><dfn data-x-href="https://tc39.es/proposal-json-modules/#sec-createsyntheticmodule">CreateSyntheticModule</dfn></li>
-     <li><dfn data-x-href="https://tc39.es/proposal-json-modules/#sec-setsyntheticmoduleexport">SetSyntheticModuleExport</dfn></li>
-     <li><dfn data-x-href="https://tc39.es/proposal-json-modules/#sec-synthetic-module-records">Synthetic Module Record</dfn></li>
+     <li>The <dfn data-x-href="https://tc39.es/proposal-json-modules/#sec-create-default-export-synthetic-module">CreateDefaultExportSyntheticModule</dfn> abstract operation</li>
+     <li>The <dfn data-x-href="https://tc39.es/proposal-json-modules/#sec-setsyntheticmoduleexport">SetSyntheticModuleExport</dfn> abstract operation</li>
+     <li>The <dfn data-x-href="https://tc39.es/proposal-json-modules/#sec-synthetic-module-records">Synthetic Module Record</dfn> specification type</li>
+     <li>The <dfn data-x-href="https://tc39.es/proposal-json-modules/#sec-parse-json-module">ParseJSONModule</dfn> abstract operation</li>
     </ul>
    </dd>
 
@@ -58396,6 +58398,9 @@ interface <dfn interface>HTMLScriptElement</dfn> : <span>HTMLElement</span> {
   <p>The contents of the external script resource for <span data-x="CSS module script">CSS module
   scripts</span> must conform to the requirements of the CSS specification. <ref spec=CSS></p>
 
+  <p>The contents of the external script resource for <span data-x="JSON module script">JSON module
+  scripts</span> must conform to the requirements of the JSON specification <ref spec=JSON>.</p>
+
   <p>When used to include <span data-x="data block">data blocks</span>, the data must be embedded
   inline, the format of the data must be given using the <code data-x="attr-script-type">type</code>
   attribute, and the contents of the <code>script</code> element must conform to the requirements
@@ -58714,7 +58719,29 @@ o............A....e
    where this <code>script</code> element appears in the document, it will not be evaluated until
    both document parsing has complete and its dependency (<code data-x="">dom-utils.mjs</code>) has
    been fetched and evaluated.</p>
+  </div>
 
+  <div class="example" id="json-module-script-example">
+   <p>The following sample shows how a <span>JSON module script</span> can be imported from inside
+   a <span>JavaScript module script</span>:</p>
+
+   <pre><code class="html" data-x="">&lt;script type="module">
+ import peopleInSpace from "http://api.open-notify.org/astros.json" assert { type: "json" };
+
+ const list = document.querySelector("#people-in-space");
+ for (const { craft, name } of peopleInSpace.people) {
+   const li = document.createElement("li");
+   li.textContent = `${name} / ${craft}`;
+   list.append(li);
+ }
+&lt;/script></code></pre>
+
+   <p>MIME type checking for module scripts is strict. In order for the fetch of the <span>JSON
+   module script</span> to succeed, the HTTP reponse must have a <span>JSON MIME type</span>, for
+   example <code data-x="">Content-Type: text/json</code>. On the other hand, if the <code
+   data-x="">assert { type: "json" }</code> part of the statement is omitted, it is assumed that the
+   intent is to import a <span>JavaScript module script</span>, and the fetch will fail if the HTTP
+   response has a MIME type that is not a <span>JavaScript MIME type</span>.</p>
   </div>
 
   <div w-nodev>
@@ -90154,8 +90181,8 @@ document.querySelector("button").addEventListener("click", bound);
      <li><p>a <span>Source Text Module Record</span>, for <span data-x="JavaScript module
      script">JavaScript module scripts</span>;</p></li>
 
-     <li><p>a <span>Synthetic Module Record</span>, for <span data-x="CSS module script">CSS
-     module scripts</span>; or</p></li>
+     <li><p>a <span>Synthetic Module Record</span>, for <span data-x="CSS module script">CSS module
+     scripts</span> and <span data-x="JSON module script">JSON module scripts</span></p></li>
 
      <li><p>null, representing a parsing failure.</p></li>
     </ul>
@@ -90221,7 +90248,7 @@ document.querySelector("button").addEventListener("click", bound);
   data-x="concept-script">script</span>. It has no additional <span data-x="struct
   item">items</span>.</p>
 
-  <p><span data-x="module script">Module scripts</span> can be classified into two types:</p>
+  <p><span data-x="module script">Module scripts</span> can be classified into three types:</p>
 
   <ul>
    <li><p>A <span>module script</span> is a <dfn data-export="">JavaScript module script</dfn> if
@@ -90233,23 +90260,33 @@ document.querySelector("button").addEventListener("click", bound);
     data-x="concept-script-record">record</span> is a <span>Synthetic Module Record</span>, and it
     was created via the <span data-x="creating a CSS module script">create a CSS module
     script</span> algorithm. CSS module scripts represent a parsed CSS stylesheet.</p>
-    <!--
-     This definition is not super-rigorous, but it doesn't need to be for now. Technically, the
-     "it was created via..." requirement is redundant, since there are no other Synthetic Module
-     Record users.
 
-     If we ever: (a) get more Synthetic Module Record Users; or (b) start testing if something is a
-     CSS module script in algorithms, instead of just referring to the concept, then we should
-     consider adding a type item to the module script struct.
+    <!--
+     This definition is not super-rigorous, but it doesn't need to be for now. If we ever start
+     testing if something is a CSS module script in algorithms, instead of just referring to the
+     concept, then we should consider adding a type item to the module script struct.
     -->
-
-    <p class="note">As CSS stylesheets do not import dependent modules, and do not throw exceptions
-    on evaluation, the <span data-x="concept-script-script-fetch-options">fetch options</span> and
-    <span data-x="concept-script-base-url">base URL</span> of a <span>CSS module script</span> are
-    always null.</p>
    </li>
+
+   <li>
+    <p>A <span>module script</span> is a <dfn data-export="">JSON module script</dfn> if its <span
+    data-x="concept-script-record">record</span> is a <span>Synthetic Module Record</span>, and it
+    was created via the <span data-x="creating a JSON module script">create a JSON module
+    script</span> algorithm. JSON module scripts represent a parsed JSON document.</p>
+
+    <!--
+     This definition is not super-rigorous, but it doesn't need to be for now. If we ever start
+     testing if something is a JSON module script in algorithms, instead of just referring to the
+     concept, then we should consider adding a type item to the module script struct.
+    -->
   </ul>
 
+  <p class="note">As CSS stylesheets and JSON documents do not import dependent modules, and do not
+  throw exceptions on evaluation, the <span data-x="concept-script-script-fetch-options">fetch
+  options</span> and <span data-x="concept-script-base-url">base URL</span> of <span data-x="CSS
+  module script">CSS module scripts</span> and <span data-x="JSON module script">JSON module
+  scripts</span> and are always null.</p>
+
   <p>The <dfn>active script</dfn> is determined by the following algorithm:</p>
 
   <ol>
@@ -90648,8 +90685,9 @@ document.querySelector("button").addEventListener("click", bound);
    type</var> be <var>entry</var>.[[Value]]. Otherwise let <var>module type</var> be "<code
    data-x="">javascript</code>".</p></li>
 
-   <li><p>If <var>module type</var> is neither "<code data-x="">javascript</code>" nor "<code
-   data-x="">css</code>", then asynchronously complete this algorithm with null, and return.</p></li>
+   <li><p>If <var>module type</var> is not "<code data-x="">javascript</code>", "<code
+   data-x="">css</code>", or "<code data-x="">json</code>", then asynchronously complete this
+   algorithm with null, and return.</p></li>
 
    <li><p><span>Fetch a single module script</span> given <var>url</var>, <var>settings
    object</var>, "<code data-x="">script</code>", <var>options</var>, <var>settings object</var>,
@@ -91126,11 +91164,12 @@ document.querySelector("button").addEventListener("click", bound);
     </ol>
    </li>
 
-   <li><p>Assert: <var>module type</var> is either "<code data-x="">javascript</code>" or "<code
-   data-x="">css</code>". Otherwise we would not have reached this point because a failure would
-   have been raised when inspecting <var>moduleRequest</var>.[[Assertions]] in <a
-   href="#validate-requested-module-specifiers">create a JavaScript module script</a> or <span>fetch
-   an import() module script graph</span>.</p></li>
+   <li><p>Assert: <var>module type</var> is "<code data-x="">javascript</code>", "<code
+   data-x="">css</code>", or "<code data-x="">json</code>". Otherwise we would not have reached
+   this point because a failure would have been raised when inspecting
+   <var>moduleRequest</var>.[[Assertions]] in <a
+   href="#validate-requested-module-specifiers">create a JavaScript module script</a> or
+   <span>fetch an import() module script graph</span>.</p></li>
 
    <li><p>Let <var>moduleMap</var> be <var>module map settings object</var>'s <span
    data-x="concept-settings-object-module-map">module map</span>.</p></li>
@@ -91215,6 +91254,11 @@ document.querySelector("button").addEventListener("click", bound);
    result of <span>creating a CSS module script</span> given <var>source text</var> and <var>module
    map settings object</var>.</p></li>
 
+   <li><p>If <var>MIME type essence</var> is a <span>JSON MIME type</span> and <var>module
+   type</var> is "<code data-x="">json</code>", then set <var>module script</var> to the result of
+   <span>creating a JSON module script</span> given <var>source text</var> and <var>module map
+   settings object</var>.</p></li>
+
    <li>
     <p><span data-x="map set">Set</span> <var>moduleMap</var>[(<var>url</var>, <var>module
     type</var>)] to <var>module script</var>, and asynchronously complete this algorithm with
@@ -91417,8 +91461,9 @@ document.querySelector("button").addEventListener("click", bound);
      data-x="">javascript</code>".</p></li>
 
      <li>
-      <p>If <var>url</var> is failure, or if <var>module type</var> is neither "<code
-      data-x="">javascript</code>" nor "<code data-x="">css</code>", then:</p>
+      <p>If <var>url</var> is failure, or if <var>module type</var> is not "<code
+      data-x="">javascript</code>", "<code data-x="">css</code>", or "<code data-x="">json</code>",
+      then:</p>
       <ol>
        <li><p>Let <var>error</var> be a new <code>TypeError</code> exception.</p></li>
 
@@ -91480,27 +91525,38 @@ document.querySelector("button").addEventListener("click", bound);
    </li>
 
    <li><p>Set <var>script</var>'s <span data-x="concept-script-record">record</span> to the result
-   of <span data-x="create a synthetic module record with a default export">creating a synthetic
-   module record with a default export</span> of <var>sheet</var> with <var>settings</var>.</p>
-   </li>
+   of <span>CreateDefaultExportSyntheticModule</span>(<var>sheet</var>).</p></li>
 
    <li><p>Return <var>script</var>.</p></li>
   </ol>
 
-  <p>To <dfn>create a synthetic module record with a default export</dfn> of a JavaScript value
-  <var>value</var> with an <span>environment settings object</span> <var>settings</var>:</p>
+  <p>To <dfn data-x="creating a JSON module script">create a JSON module script</dfn>, given a
+  string <var>source</var> and an <span>environment settings object</span> <var>settings</var>:</p>
 
   <ol>
+   <li><p>Let <var>script</var> be a new <span>module script</span> that this algorithm will
+   subsequently initialize.</p></li>
+
+   <li><p>Set <var>script</var>'s <span>settings object</span> to <var>settings</var>.</p></li>
+
+   <li><p>Set <var>script</var>'s <span data-x="concept-script-base-url">base URL</span> and
+   <span data-x="concept-script-script-fetch-options">fetch options</span> to null.</p></li>
+
+   <li><p>Set <var>script</var>'s <span data-x="concept-script-parse-error">parse error</span> and
+   <span data-x="concept-script-error-to-rethrow">error to rethrow</span> to null.</p></li>
+
    <li>
-    <p>Return <span>CreateSyntheticModule</span>(« "<code data-x="">default</code>" », the following
-    steps, <var>settings</var>'s <span data-x="environment settings object's Realm">Realm</span>,
-    <var>value</var>) with the following steps given <var>module</var> as an argument:</p>
+    <p>Let <var>result</var> be <span>ParseJSONModule</span>(<var>source</var>).</p>
 
-    <ol>
-     <li>Perform ! <span>SetSyntheticModuleExport</span>(<var>module</var>, "<code
-     data-x="">default</code>", <var>module</var>.[[HostDefined]]).</li>
-    </ol>
+    <p>If this throws an exception, set <var>script</var>'s <span
+    data-x="concept-script-parse-error">parse error</span> to that exception, and return
+    <var>script</var>.</p>
    </li>
+
+   <li><p>Set <var>script</var>'s <span data-x="concept-script-record">record</span> to
+   <var>result</var>.</p></li>
+
+   <li><p>Return <var>script</var>.</p></li>
   </ol>
 
   <h5 id="calling-scripts">Calling scripts</h5>