Upgrade to ESLint 9+ and flat config (Leaflet#9410)

build/integrity.mjs

@@ -8,7 +8,7 @@ import ssri from 'ssri';
 // See: https://rollupjs.org/guide/en/#importing-packagejson
 const {version} = JSON.parse(readFileSync(new URL('../package.json', import.meta.url)));
 
-const getIntegrity = async path => new Promise((resolve) => {
+const getIntegrity = path => new Promise((resolve) => {
 	https.get(`https://unpkg.com/leaflet@${version}/dist/${path}`, (res) => {
 		ssri.fromStream(res, {algorithms: ['sha256']}).then(integrity => resolve(integrity.toString()));
 	});

docs/docs/js/docs.js

@@ -1,4 +1,4 @@
-/* global hljs */
+/* global hljs, L */
 hljs.configure({tabReplace: '    '});
 hljs.initHighlighting();
 

docs/examples/wms/wms-example1.md

@@ -13,4 +13,4 @@ title: WMS example
 		layers: 'TOPO-OSM-WMS'
 	}).addTo(map);
 
-</script>
+</script>

eslint.config.mjs

@@ -0,0 +1,83 @@
+import config from 'eslint-config-mourner';
+import scriptTags from '@mapbox/eslint-plugin-script-tags';
+import importPlugin from 'eslint-plugin-import-x';
+import globals from 'globals';
+
+export default [
+	...config,
+	{
+		files: ['*.js', '*.mjs', '*.cjs'],
+	},
+	{
+		ignores: [
+			'dist',
+			'docs/docs/highlight',
+			'docs/examples/choropleth/us-states.js',
+			'docs/examples/geojson/sample-geojson.js',
+			'docs/examples/map-panes/eu-countries.js',
+			'docs/examples/extending/extending-2-layers.md',
+			'docs/_posts/201*',
+			'docs/_site',
+			'coverage'
+		]
+	},
+	{
+		plugins: {
+			import: importPlugin
+		},
+		rules: {
+			'dot-notation': 'off',
+			'consistent-return': 'off',
+			'curly': 'error',
+			'no-unused-expressions': ['error', {allowShortCircuit: true}],
+			'no-unused-vars': ['error', {caughtErrors: 'none'}],
+
+			'import/extensions': ['error', 'ignorePackages'],
+
+			'@stylistic/js/indent': ['error', 'tab', {VariableDeclarator: 0, flatTernaryExpressions: true}],
+			'@stylistic/js/no-mixed-spaces-and-tabs': ['error', 'smart-tabs'],
+			'@stylistic/js/key-spacing': 'off',
+			'@stylistic/js/linebreak-style': ['off', 'unix'],
+			'@stylistic/js/spaced-comment': 'error',
+
+			// TODO: Re-enable the rules below and fix the linting issues.
+			'no-invalid-this': 'off',
+			'prefer-object-has-own': 'error',
+			'prefer-spread': 'off',
+			'no-new': 'off'
+		}
+	},
+	{
+		files: ['spec/**'],
+		languageOptions: {
+			globals: {...globals.mocha}
+		},
+		rules: {
+			'no-unused-expressions': 'off'
+		}
+	},
+	{
+		files: ['build/**'],
+		languageOptions: {
+			parserOptions: {
+				ecmaVersion: 2022
+			}
+		}
+	},
+	{
+		files: ['docs/**/*.md'],
+		plugins: {
+			scriptTags: {
+				processors: {md: scriptTags.processors['.md']}
+			}
+		},
+		processor: 'scriptTags/md',
+		rules: {
+			'no-unused-vars': 'off',
+			'@stylistic/js/eol-last': 'off'
+		},
+		languageOptions: {
+			globals: {L: false}
+		}
+	}
+];