Make the data property of MessageEvent generic (#860)

Co-authored-by: Nathan Shively-Sanders <293473+sandersn@users.noreply.github.com>

Author: cawa-93

baselines/dom.generated.d.ts

@@ -805,8 +805,8 @@ interface MediaTrackSupportedConstraints {
     width?: boolean;
 }
 
-interface MessageEventInit extends EventInit {
-    data?: any;
+interface MessageEventInit<T = any> extends EventInit {
+    data?: T;
     lastEventId?: string;
     origin?: string;
     ports?: MessagePort[];
@@ -10395,11 +10395,11 @@ declare var MessageChannel: {
 };
 
 /** A message received by a target object. */
-interface MessageEvent extends Event {
+interface MessageEvent<T = any> extends Event {
     /**
      * Returns the data of the message.
      */
-    readonly data: any;
+    readonly data: T;
     /**
      * Returns the last event ID string, for server-sent events.
      */
@@ -10420,7 +10420,7 @@ interface MessageEvent extends Event {
 
 declare var MessageEvent: {
     prototype: MessageEvent;
-    new(type: string, eventInitDict?: MessageEventInit): MessageEvent;
+    new<T>(type: string, eventInitDict?: MessageEventInit<T>): MessageEvent<T>;
 };
 
 interface MessagePortEventMap {

baselines/webworker.generated.d.ts

@@ -262,8 +262,8 @@ interface KeyAlgorithm {
     name: string;
 }
 
-interface MessageEventInit extends EventInit {
-    data?: any;
+interface MessageEventInit<T = any> extends EventInit {
+    data?: T;
     lastEventId?: string;
     origin?: string;
     ports?: MessagePort[];
@@ -2221,11 +2221,11 @@ declare var MessageChannel: {
 };
 
 /** A message received by a target object. */
-interface MessageEvent extends Event {
+interface MessageEvent<T = any> extends Event {
     /**
      * Returns the data of the message.
      */
-    readonly data: any;
+    readonly data: T;
     /**
      * Returns the last event ID string, for server-sent events.
      */
@@ -2246,7 +2246,7 @@ interface MessageEvent extends Event {
 
 declare var MessageEvent: {
     prototype: MessageEvent;
-    new(type: string, eventInitDict?: MessageEventInit): MessageEvent;
+    new<T>(type: string, eventInitDict?: MessageEventInit<T>): MessageEvent<T>;
 };
 
 interface MessagePortEventMap {

inputfiles/overridingTypes.json

@@ -2696,6 +2696,29 @@
                     }
                 }
             },
+            "MessageEvent": {
+                "name": "MessageEvent",
+                "properties": {
+                    "property": {
+                        "data": {
+                            "name": "data",
+                            "read-only": 1,
+                            "override-type": "T"
+                        }
+                    }
+                },
+                "type-parameters": [
+                    {
+                        "name": "T",
+                        "default": "any"
+                    }
+                ],
+                "constructor": {
+                    "override-signatures": [
+                        "new<T>(type: string, eventInitDict?: MessageEventInit<T>): MessageEvent<T>"
+                    ]
+                }
+            },
             "Clients": {
                 "methods": {
                     "method": {
@@ -2711,6 +2734,24 @@
     },
     "dictionaries": {
         "dictionary": {
+            "MessageEventInit": {
+                "name": "MessageEventInit",
+                "members": {
+                    "member": {
+                        "data": {
+                            "name": "data",
+                            "override-type": "T",
+                            "required": 0
+                        }
+                    }
+                },
+                "type-parameters": [
+                    {
+                        "name": "T",
+                        "default": "any"
+                    }
+                ]
+            },
             "CustomEventInit": {
                 "name": "CustomEventInit",
                 "members": {