w3c · saschanaz · Jul 23, 2025 · Mar 26, 2024 · Jan 24, 2025 · May 1, 2025
diff --git a/README.md b/README.md
@@ -186,6 +186,7 @@ properties:
 * `sourceName`: the source name you passed to `parse()`.
 * `level`: `"error"` by default, can be `"warning"` for some validations for e.g. potential future deprecations.
 * `ruleName`: Only for validations. Currently the followings are supported:
+   * `async-iterable-idl-to-js`: `async_iterable` types cannot be returned from the IDL to JS.
    * `attr-invalid-type`: Attributes cannot have sequences, records, nor dictionaries.
    * `dict-arg-default`: Optional dictionary type arguments must have a default value of `{}`.
    * `dict-arg-optional`: Dictionary type arguments must be optional if the type does not include a required field.
@@ -236,7 +237,7 @@ attached to a field called `idlType`:
 Where the fields are as follows:
 
 * `type`: String indicating where this type is used. Can be `null` if not applicable.
-* `generic`: String indicating the generic type (e.g. "Promise", "sequence").
+* `generic`: String indicating the generic type (e.g. "Promise", "sequence", "async_sequence").
 * `idlType`: String indicating the type name, or array of subtypes if the type is
   generic or a union.
 * `nullable`: `true` if the type is nullable.

diff --git a/lib/productions/attribute.js b/lib/productions/attribute.js
@@ -75,7 +75,9 @@ export class Attribute extends Base {
     yield* this.extAttrs.validate(defs);
     yield* this.idlType.validate(defs);
 
-    if (["sequence", "record"].includes(this.idlType.generic)) {
+    if (
+      ["async_sequence", "sequence", "record"].includes(this.idlType.generic)
+    ) {
       const message = `Attributes cannot accept ${this.idlType.generic} types.`;
       yield validationError(
         this.tokens.name,

diff --git a/lib/productions/callback.js b/lib/productions/callback.js
@@ -5,6 +5,7 @@ import {
   unescape,
   autoParenter,
 } from "./helpers.js";
+import { validationError } from "../error.js";
 
 export class CallbackFunction extends Base {
   /**
@@ -44,6 +45,18 @@ export class CallbackFunction extends Base {
 
   *validate(defs) {
     yield* this.extAttrs.validate(defs);
+    for (const arg of this.arguments) {
+      yield* arg.validate(defs);
+      if (arg.idlType.generic === "async_sequence") {
+        const message = `async_sequence types cannot be returned as a callback argument.`;
+        yield validationError(
+          arg.tokens.name,
+          arg,
+          "async-sequence-idl-to-js",
+          message,
+        );
+      }
+    }
     yield* this.idlType.validate(defs);
   }
 

diff --git a/lib/productions/operation.js b/lib/productions/operation.js
@@ -68,6 +68,15 @@ export class Operation extends Base {
       yield validationError(this.tokens.open, this, "incomplete-op", message);
     }
     if (this.idlType) {
+      if (this.idlType.generic === "async_sequence") {
+        const message = `async_sequence types cannot be returned by an operation.`;
+        yield validationError(
+          this.idlType.tokens.base,
+          this,
+          "async-sequence-idl-to-js",
+          message,
+        );
+      }
       yield* this.idlType.validate(defs);
     }
     for (const argument of this.arguments) {

diff --git a/lib/productions/type.js b/lib/productions/type.js
@@ -20,6 +20,7 @@ function generic_type(tokeniser, typeName) {
     "FrozenArray",
     "ObservableArray",
     "Promise",
+    "async_sequence",
     "sequence",
     "record",
   );
@@ -42,6 +43,7 @@ function generic_type(tokeniser, typeName) {
       ret.subtype.push(subtype);
       break;
     }
+    case "async_sequence":
     case "sequence":
     case "FrozenArray":
     case "ObservableArray": {

diff --git a/lib/tokeniser.js b/lib/tokeniser.js
@@ -73,6 +73,7 @@ const nonRegexTerminals = [
   "ObservableArray",
   "Promise",
   "async_iterable",
+  "async_sequence",
   "bigint",
   "boolean",
   "byte",

diff --git a/test/invalid/baseline/async-sequence-const.txt b/test/invalid/baseline/async-sequence-const.txt
@@ -0,0 +1,3 @@
+Syntax error at line 3 in async-sequence-const.webidl, since `interface AsyncSequence`:
+  const async_sequence<long, short
+        ^ Const lacks a type
diff --git a/test/invalid/baseline/async-sequence-idl-to-js.txt b/test/invalid/baseline/async-sequence-idl-to-js.txt
@@ -0,0 +1,9 @@
+(attr-invalid-type) Validation error at line 3 in async-sequence-idl-to-js.webidl, inside `interface asyncIterableAsAttribute -> attribute invalid`:
+  attribute async_sequence<short> invalid;
+                                  ^ Attributes cannot accept async_sequence types.
+(async-sequence-idl-to-js) Validation error at line 5 in async-sequence-idl-to-js.webidl, inside `interface asyncIterableAsAttribute -> operation invalidOp`:
+  async_sequence<DOMString> invalidOp
+  ^ async_sequence types cannot be returned by an operation.
+(async-sequence-idl-to-js) Validation error at line 8 in async-sequence-idl-to-js.webidl, inside `callback DoSomething -> argument bool`:
+ (async_sequence<DOMString> bool);
+                            ^ async_sequence types cannot be returned as a callback argument.
diff --git a/test/invalid/baseline/async-sequence-two-params.txt b/test/invalid/baseline/async-sequence-two-params.txt
@@ -0,0 +1,3 @@
+Syntax error at line 3 in async-sequence-two-params.webidl, since `interface AsyncSequence`:
+  async_sequence<long, short> foo(
+                     ^ Missing closing bracket after async_sequence
diff --git a/test/invalid/idl/async-sequence-const.webidl b/test/invalid/idl/async-sequence-const.webidl
@@ -0,0 +1,4 @@
+[Exposed=Window]
+interface AsyncSequence {
+  const async_sequence<long, short>  ASYNC_SEQUENCE = 0;
+};
diff --git a/test/invalid/idl/async-sequence-idl-to-js.webidl b/test/invalid/idl/async-sequence-idl-to-js.webidl
@@ -0,0 +1,9 @@
+[Exposed=Window]
+interface asyncIterableAsAttribute {
+  attribute async_sequence<short> invalid;
+  attribute (async_sequence<short> or boolean) invalidUnion; // TODO
+  async_sequence<DOMString> invalidOp();
+};
+
+callback DoSomething = Promise<DOMString> (async_sequence<DOMString> bool);
+
diff --git a/test/invalid/idl/async-sequence-two-params.webidl b/test/invalid/idl/async-sequence-two-params.webidl
@@ -0,0 +1,4 @@
+[Exposed=Window]
+interface AsyncSequence {
+  async_sequence<long, short> foo();
+};
diff --git a/test/syntax/baseline/async-sequence.json b/test/syntax/baseline/async-sequence.json
@@ -0,0 +1,131 @@
+[
+    {
+        "type": "interface",
+        "name": "Canvas",
+        "inheritance": null,
+        "members": [
+            {
+                "type": "operation",
+                "name": "drawPolygonAsync",
+                "idlType": {
+                    "type": "return-type",
+                    "extAttrs": [],
+                    "generic": "Promise",
+                    "nullable": false,
+                    "union": false,
+                    "idlType": [
+                        {
+                            "type": "return-type",
+                            "extAttrs": [],
+                            "generic": "",
+                            "nullable": false,
+                            "union": false,
+                            "idlType": "undefined"
+                        }
+                    ]
+                },
+                "arguments": [
+                    {
+                        "type": "argument",
+                        "name": "coordinates",
+                        "extAttrs": [],
+                        "idlType": {
+                            "type": "argument-type",
+                            "extAttrs": [],
+                            "generic": "async_sequence",
+                            "nullable": false,
+                            "union": false,
+                            "idlType": [
+                                {
+                                    "type": "argument-type",
+                                    "extAttrs": [],
+                                    "generic": "",
+                                    "nullable": false,
+                                    "union": false,
+                                    "idlType": "float"
+                                }
+                            ]
+                        },
+                        "default": null,
+                        "optional": false,
+                        "variadic": false
+                    }
+                ],
+                "extAttrs": [],
+                "special": ""
+            }
+        ],
+        "extAttrs": [],
+        "partial": false
+    },
+    {
+        "type": "interface",
+        "name": "I",
+        "inheritance": null,
+        "members": [
+            {
+                "type": "operation",
+                "name": "f1",
+                "idlType": {
+                    "type": "return-type",
+                    "extAttrs": [],
+                    "generic": "Promise",
+                    "nullable": false,
+                    "union": false,
+                    "idlType": [
+                        {
+                            "type": "return-type",
+                            "extAttrs": [],
+                            "generic": "",
+                            "nullable": false,
+                            "union": false,
+                            "idlType": "undefined"
+                        }
+                    ]
+                },
+                "arguments": [
+                    {
+                        "type": "argument",
+                        "name": "arg",
+                        "extAttrs": [],
+                        "idlType": {
+                            "type": "argument-type",
+                            "extAttrs": [],
+                            "generic": "async_sequence",
+                            "nullable": false,
+                            "union": false,
+                            "idlType": [
+                                {
+                                    "type": "argument-type",
+                                    "extAttrs": [
+                                        {
+                                            "type": "extended-attribute",
+                                            "name": "XAttr",
+                                            "rhs": null,
+                                            "arguments": []
+                                        }
+                                    ],
+                                    "generic": "",
+                                    "nullable": false,
+                                    "union": false,
+                                    "idlType": "float"
+                                }
+                            ]
+                        },
+                        "default": null,
+                        "optional": false,
+                        "variadic": false
+                    }
+                ],
+                "extAttrs": [],
+                "special": ""
+            }
+        ],
+        "extAttrs": [],
+        "partial": false
+    },
+    {
+        "type": "eof",
+        "value": ""
+    }
+]
diff --git a/test/syntax/idl/async-sequence.webidl b/test/syntax/idl/async-sequence.webidl
@@ -0,0 +1,7 @@
+interface Canvas {
+  Promise<undefined> drawPolygonAsync(async_sequence<float> coordinates);
+};
+
+interface I {
+  Promise<undefined> f1(async_sequence<[XAttr] float> arg);
+};