Use explicit extensions for imports within src

.eslintrc.json

@@ -144,7 +144,8 @@
         "local/no-in-operator": "error",
         "local/debug-assert": "error",
         "local/no-keywords": "error",
-        "local/jsdoc-format": "error"
+        "local/jsdoc-format": "error",
+        "local/js-extensions": "error"
     },
     "overrides": [
         // By default, the ESLint CLI only looks at .js files. But, it will also look at

scripts/eslint/rules/js-extensions.cjs

@@ -0,0 +1,55 @@
+const { createRule } = require("./utils.cjs");
+
+/** @typedef {import("@typescript-eslint/utils").TSESTree.ImportDeclaration | import("@typescript-eslint/utils").TSESTree.ExportAllDeclaration | import("@typescript-eslint/utils").TSESTree.ExportNamedDeclaration | import("@typescript-eslint/utils").TSESTree.TSImportEqualsDeclaration} ImportOrExport */
+
+module.exports = createRule({
+    name: "js-extensions",
+    meta: {
+        docs: {
+            description: ``,
+        },
+        messages: {
+            missingJsExtension: `This relative import is missing a '.js' extension`,
+        },
+        schema: [],
+        type: "suggestion",
+        fixable: "code",
+    },
+    defaultOptions: [],
+
+    create(context) {
+        /** @type {(node: ImportOrExport) => void} */
+        const check = node => {
+            let source;
+            if (node.type === "TSImportEqualsDeclaration") {
+                const moduleReference = node.moduleReference;
+                if (
+                    moduleReference.type === "TSExternalModuleReference"
+                    && moduleReference.expression.type === "Literal"
+                    && typeof moduleReference.expression.value === "string"
+                ) {
+                    source = moduleReference.expression;
+                }
+            }
+            else {
+                source = node.source;
+            }
+
+            if (source?.value.startsWith(".") && !/\.[cm]?js$/.test(source.value)) {
+                const quote = source.raw[0];
+                context.report({
+                    messageId: "missingJsExtension",
+                    node: source,
+                    fix: fixer => fixer.replaceText(source, `${quote}${source.value}.js${quote}`),
+                });
+            }
+        };
+
+        return {
+            ImportDeclaration: check,
+            ExportAllDeclaration: check,
+            ExportNamedDeclaration: check,
+            TSImportEqualsDeclaration: check,
+        };
+    },
+});

scripts/eslint/tests/js-extensions.cjs

@@ -0,0 +1,149 @@
+const { RuleTester } = require("./support/RuleTester.cjs");
+const rule = require("../rules/js-extensions.cjs");
+
+const ruleTester = new RuleTester({
+    parserOptions: {
+        warnOnUnsupportedTypeScriptVersion: false,
+    },
+    parser: require.resolve("@typescript-eslint/parser"),
+});
+
+ruleTester.run("js-extensions", rule, {
+    valid: [
+        {
+            code: `
+import "some-library";
+import "./a.js";
+import "./a.mjs";
+import "./a.cjs";
+import "../foo/a.js";
+import "../foo/a.mjs";
+import "../foo/a.cjs";
+        `,
+        },
+        {
+            code: `
+import * as blah from "some-library";
+import * as blah from "./a.js";
+import * as blah from "./a.mjs";
+import * as blah from "./a.cjs";
+import * as blah from "../foo/a.js";
+import * as blah from "../foo/a.mjs";
+import * as blah from "../foo/a.cjs";
+        `,
+        },
+        {
+            code: `
+export * from "some-library";
+export * from "./a.js";
+export * from "./a.mjs";
+export * from "./a.cjs";
+export * from "../foo/a.js";
+export * from "../foo/a.mjs";
+export * from "../foo/a.cjs";
+        `,
+        },
+        {
+            code: `
+import blah = require("some-library");
+import blah = require("./a.js");
+import blah = require("./a.mjs");
+import blah = require("./a.cjs");
+import blah = require("../foo/a.js");
+import blah = require("../foo/a.mjs");
+import blah = require("../foo/a.cjs");
+        `,
+        },
+    ],
+
+    invalid: [
+        {
+            code: `
+import "./a";
+import "../foo/a";
+        `,
+            errors: [
+                {
+                    messageId: "missingJsExtension",
+                    line: 2,
+                    column: 8,
+                },
+                {
+                    messageId: "missingJsExtension",
+                    line: 3,
+                    column: 8,
+                },
+            ],
+            output: `
+import "./a.js";
+import "../foo/a.js";
+        `,
+        },
+        {
+            code: `
+import * as blah from "./a";
+import * as blah from "../foo/a";
+        `,
+            errors: [
+                {
+                    messageId: "missingJsExtension",
+                    line: 2,
+                    column: 23,
+                },
+                {
+                    messageId: "missingJsExtension",
+                    line: 3,
+                    column: 23,
+                },
+            ],
+            output: `
+import * as blah from "./a.js";
+import * as blah from "../foo/a.js";
+        `,
+        },
+        {
+            code: `
+export * from "./a";
+export * from "../foo/a";
+        `,
+            errors: [
+                {
+                    messageId: "missingJsExtension",
+                    line: 2,
+                    column: 15,
+                },
+                {
+                    messageId: "missingJsExtension",
+                    line: 3,
+                    column: 15,
+                },
+            ],
+            output: `
+export * from "./a.js";
+export * from "../foo/a.js";
+        `,
+        },
+        {
+            code: `
+import blah = require("./a");
+import blah = require("../foo/a");
+        `,
+            errors: [
+                {
+                    messageId: "missingJsExtension",
+                    line: 2,
+                    column: 23,
+                },
+                {
+                    messageId: "missingJsExtension",
+                    line: 3,
+                    column: 23,
+                },
+            ],
+            output: `
+import blah = require("./a.js");
+import blah = require("../foo/a.js");
+        `,
+        },
+    ],
+});