CHANGELOG.md

 # html-validate changelog
 
+# [2.13.0](https://gitlab.com/html-validate/html-validate/compare/v2.12.0...v2.13.0) (2020-02-02)
+
+### Features
+
+- **meta:** allow plugins to add copyable metadata ([242eaa8](https://gitlab.com/html-validate/html-validate/commit/242eaa882afb71e527b07a2a92e6d45adf4f02e7))
+
 # [2.12.0](https://gitlab.com/html-validate/html-validate/compare/v2.11.0...v2.12.0) (2020-01-27)
 
 ### Bug Fixes

docs/dev/writing-plugins.md

@@ -277,3 +277,53 @@ switch (meta.myProperty) {
   default: /* ... */
 }
 ```
+
+### Copyable metadata
+
+Plugins leveraging usage of `loadMeta` for advanced handling of metadata loading must explicitly mark the copyable properties as `copyable`:
+
+```js
+module.exports = {
+  elementSchema: {
+    properties: {
+      foo: {
+        copyable: true,
+      },
+      bar: {
+        copyable: false,
+      },
+    },
+  },
+};
+```
+
+Given these two properties only `foo` will be copied (loaded) onto the element when using `loadMeta`:
+
+```js
+module.exports = {
+  "my-element": {
+    foo: "original",
+    bar: "original",
+  },
+  "my-element:slot": {
+    foo: "overwritten",
+    bar: "overwritten",
+  },
+};
+```
+
+```js
+function processElement(node) {
+  const meta = this.getMetaFor("my-element:slot");
+  node.loadMeta(meta);
+}
+```
+
+The resulting metadata will now be:
+
+```json
+{
+  "foo": "overwritten",
+  "bar": "original"
+}
+```

package.json

 {
   "name": "html-validate",
-  "version": "2.12.0",
+  "version": "2.13.0",
   "description": "html linter",
   "keywords": [
     "html",
@@ -94,8 +94,8 @@
     "minimist": "^1.2.0"
   },
   "devDependencies": {
-    "@babel/core": "7.8.3",
-    "@babel/preset-env": "7.8.3",
+    "@babel/core": "7.8.4",
+    "@babel/preset-env": "7.8.4",
     "@commitlint/cli": "8.3.5",
     "@html-validate/commitlint-config": "1.0.1",
     "@html-validate/eslint-config": "1.0.8",
@@ -105,10 +105,10 @@
     "@types/estree": "0.0.42",
     "@types/glob": "7.1.1",
     "@types/inquirer": "6.5.0",
-    "@types/jest": "24.9.1",
+    "@types/jest": "25.1.1",
     "@types/json-merge-patch": "0.0.4",
     "@types/minimist": "1.2.0",
-    "@types/node": "11.15.4",
+    "@types/node": "11.15.5",
     "autoprefixer": "9.7.4",
     "babelify": "10.0.0",
     "bootstrap-sass": "3.4.1",
@@ -138,15 +138,15 @@
     "jest-diff": "25.1.0",
     "jest-junit": "10.0.0",
     "jquery": "3.4.1",
-    "lint-staged": "10.0.2",
+    "lint-staged": "10.0.7",
     "load-grunt-tasks": "5.1.0",
     "minimatch": "3.0.4",
     "prettier": "1.19.1",
     "sass": "1.25.0",
-    "semantic-release": "16.0.3",
+    "semantic-release": "17.0.2",
     "serve-static": "1.14.1",
     "strip-ansi": "6.0.0",
-    "ts-jest": "25.0.0",
+    "ts-jest": "25.1.0",
     "tslint": "5.20.1",
     "tslint-config-prettier": "1.18.0",
     "typescript": "3.7.5"

src/config/config.ts

@@ -5,7 +5,7 @@ import path from "path";
 import { Source } from "../context";
 import { NestedError, SchemaValidationError } from "../error";
 import { MetaTable } from "../meta";
-import { MetaDataTable } from "../meta/element";
+import { MetaCopyableProperty, MetaDataTable } from "../meta/element";
 import { Plugin } from "../plugin";
 import schema from "../schema/config.json";
 import { TransformContext, Transformer, TRANSFORMER_API } from "../transform";
@@ -174,6 +174,7 @@ export class Config {
 		/* load plugins */
 		this.plugins = this.loadPlugins(this.config.plugins || []);
 		this.configurations = this.loadConfigurations(this.plugins);
+		this.extendMeta(this.plugins);
 
 		/* process extended configs */
 		for (const extend of this.config.extends) {
@@ -394,6 +395,25 @@ export class Config {
 		return configs;
 	}
 
+	private extendMeta(plugins: LoadedPlugin[]): void {
+		for (const plugin of plugins) {
+			if (!plugin.elementSchema) {
+				continue;
+			}
+
+			const { properties } = plugin.elementSchema;
+			if (!properties) {
+				continue;
+			}
+
+			for (const [key, schema] of Object.entries(properties)) {
+				if (schema.copyable && !MetaCopyableProperty.includes(key)) {
+					MetaCopyableProperty.push(key);
+				}
+			}
+		}
+	}
+
 	/**
 	 * Transform a source.
 	 *

src/plugin/plugin.spec.ts

 import { Config } from "../config";
 import { ConfigError } from "../config/error";
 import { Source } from "../context";
+import { HtmlElement } from "../dom";
 import { Engine } from "../engine";
 import { EventHandler } from "../event";
 import { Parser } from "../parser";
@@ -236,6 +237,45 @@ describe("Plugin", () => {
 			});
 			expect(() => config.getMetaTable()).not.toThrow();
 		});
+
+		it("should support copyable properties", () => {
+			mockPlugin.elementSchema = {
+				properties: {
+					foo: {
+						copyable: true,
+					},
+					bar: {
+						copyable: false,
+					},
+				},
+			};
+			config = Config.fromObject({
+				plugins: ["mock-plugin"],
+				elements: [
+					{
+						"my-element": {
+							foo: "original",
+							bar: "original",
+						},
+						"my-element:real": {
+							foo: "copied",
+							bar: "copied",
+						},
+					},
+				],
+			});
+			const metaTable = config.getMetaTable();
+			const a = metaTable.getMetaFor("my-element");
+			const b = metaTable.getMetaFor("my-element:real");
+			const node = new HtmlElement("my-element", null, null, a);
+			node.loadMeta(b);
+			expect(node.meta).toEqual({
+				tagName: "my-element",
+				foo: "copied" /* foo is marked for copying */,
+				bar: "original" /* bar is not marked for copying */,
+				void: false /* autofilled */,
+			});
+		});
 	});
 
 	describe("callbacks", () => {