AssemblyScript · jtenner · Mar 27, 2026 · Mar 27, 2026 · Mar 27, 2026 · Mar 27, 2026
diff --git a/.eslintrc.cjs b/.eslintrc.cjs
@@ -0,0 +1,274 @@
+/* global module */
+
+module.exports = {
+  root: true,
+  ignorePatterns: [
+    // These fixtures intentionally exercise AssemblyScript-only syntax that
+    // TypeScript's parser rejects before lint rules can run.
+    "tests/compiler/parameter-decorators.ts",
+    "tests/transform/parameter-decorators.ts"
+  ],
+  parser: "@typescript-eslint/parser",
+  plugins: [
+    "@typescript-eslint",
+  ],
+  extends: [
+    "eslint:recommended",
+    "plugin:@typescript-eslint/eslint-recommended",
+    "plugin:@typescript-eslint/recommended",
+  ],
+  parserOptions: {
+    ecmaVersion: 2020,
+    sourceType: "module",
+    ecmaFeatures: {}
+  },
+  globals: {
+    "globalThis": "readonly",
+    "BigInt64Array": "readonly",
+    "BigUint64Array": "readonly",
+    "WebAssembly": "readonly",
+    "FinalizationRegistry": "readonly",
+    "fetch": "readonly",
+    "URL": "readonly",
+    "console": "readonly"
+  },
+
+  // === General rules =========================================================
+
+  rules: {
+    // Omitted semicolons are hugely popular, yet within the compiler it makes
+    // sense to be better safe than sorry.
+    "semi": "error",
+
+    // Our code bases uses 2 spaces for indentation, and we enforce it here so
+    // files don't mix spaces, tabs or different indentation levels.
+    "indent": ["error", 2, {
+      "SwitchCase": 1,
+      "VariableDeclarator": "first",
+      "offsetTernaryExpressions": true,
+      "ignoredNodes": [ // FIXME: something's odd here
+        "ConditionalExpression > *",
+        "ConditionalExpression > * > *",
+        "ConditionalExpression > * > * > *"
+      ]
+    }],
+
+    // This is mostly visual style, making comments look uniform.
+    "spaced-comment": ["error", "always", {
+      "markers": ["/"],   // triple-slash
+      "exceptions": ["/"] // all slashes
+    }],
+
+    // This tends to be annoying as it encourages developers to make everything
+    // that is never reassigned a 'const', sometimes semantically incorrect so,
+    // typically leading to huge diffs in follow-up PRs modifying affected code.
+    "prefer-const": "off",
+
+    // It is perfectly fine to declare top-level variables with `var`, yet this
+    // rule doesn't provide configuration options that would help.
+    "no-var": "off",
+
+    // Quite often, dealing with multiple related cases at once or otherwise
+    // falling through is exactly the point of using a switch.
+    "no-fallthrough": "off",
+
+    // Typical false-positives here are `do { ... } while (true)` statements or
+    // similar, but the only option provided here is not checking any loops.
+    "no-constant-condition": ["error", { checkLoops: false }],
+
+    // Functions are nested in blocks occasionally, and there haven't been any
+    // problems with this so far, so turning the check off.
+    "no-inner-declarations": "off",
+
+    // Quite common in scenarios where an iteration starts at `current = this`.
+    "@typescript-eslint/no-this-alias": "off",
+
+    // Interferes with tests and 64-bit literals
+    "@typescript-eslint/no-loss-of-precision": "off",
+
+    // Disabled here, but enabled again for JavaScript files.
+    "no-unused-vars": "off",
+
+    // Disabled here, but enabled again for TypeScript files.
+    "@typescript-eslint/no-unused-vars": "off"
+  },
+  overrides: [
+
+    // === JavaScript rules ====================================================
+
+    {
+      env: {
+        "browser": true,
+        "amd": true,
+        "node": true,
+        "es6": true
+      },
+      files: [
+        "**/*.js",
+        "bin/*"
+      ],
+      rules: {
+        // We are testing both ESM and UMD, so don't limit us.
+        "@typescript-eslint/no-var-requires": "off",
+
+        // This rule does not behave well in JS files.
+        "@typescript-eslint/explicit-module-boundary-types": "off",
+
+        // Enforcing to remove function parameters on stubs makes code less
+        // maintainable, so we instead allow unused function parameters.
+        "no-unused-vars": [
+          "warn", {
+            "vars": "local",
+            "args": "none",
+            "ignoreRestSiblings": false
+          }
+        ],
+
+        "@typescript-eslint/no-loss-of-precision": "error",
+      }
+    },
+
+    // === TypeScript rules ====================================================
+
+    {
+      files: [
+        "**/*.ts"
+      ],
+      rules: {
+        // Enforcing to remove function parameters on stubs makes code less
+        // maintainable, so we instead allow unused function parameters.
+        "@typescript-eslint/no-unused-vars": [
+          "warn", {
+            "vars": "local",
+            "varsIgnorePattern": "^[A-Z](?:From|To)?$", // ignore type params
+            "args": "none",
+            "ignoreRestSiblings": false
+          }
+        ]
+      }
+    },
+
+    // === AssemblyScript rules (extends TypeScript rules) =====================
+
+    {
+      files: [
+        "**/assembly/**/*.ts",
+        "src/**/*.ts",
+        "lib/parse/src/**/*.ts"
+      ],
+      rules: {
+        // Namespaces are quite useful in AssemblyScript
+        "@typescript-eslint/no-namespace": "off",
+
+        // There is actually codegen difference here
+        "@typescript-eslint/no-array-constructor": "off",
+
+        // Sometimes it can't be avoided to add a @ts-ignore
+        "@typescript-eslint/ban-ts-comment": "off",
+
+        // Utilized to achieve portability in some cases
+        "@typescript-eslint/no-non-null-assertion": "off",
+      }
+    },
+
+    // === Compiler rules (extends AssemblyScript rules) =======================
+
+    {
+      files: [
+        "src/**/*.ts",
+        "std/assembly/**/*.ts"
+      ],
+      rules: {
+        // There is an actual codegen difference here - TODO: revisit
+        "no-cond-assign": "off",
+
+        // Not all types can be omitted in AS yet - TODO: revisit
+        "@typescript-eslint/no-inferrable-types": "off",
+
+        // Used rarely to reference internals that are not user-visible
+        "@typescript-eslint/triple-slash-reference": "off",
+
+        // The compiler has its own `Function` class for example
+        "no-shadow-restricted-names": "off",
+        "@typescript-eslint/ban-types": "off"
+      }
+    },
+
+    // === Standard Library rules (extends AssemblyScript rules) ===============
+
+    {
+      files: [
+        "std/assembly/**/*.ts"
+      ],
+      rules: {
+        // We are implementing with --noLib, so we shadow all the time
+        "no-shadow-restricted-names": "off",
+
+        // Similarly, sometimes we need the return type to be String, not string
+        "@typescript-eslint/ban-types": "off"
+      }
+    },
+
+    // === Standard Definition rules (extends TypeScript rules) ================
+
+    {
+      files: [
+        "std/**/*.d.ts"
+      ],
+      rules: {
+        // Often required to achieve compatibility with TypeScript
+        "@typescript-eslint/no-explicit-any": "off",
+
+        // Interfaces can be stubs here, i.e. not yet fully implemented
+        "@typescript-eslint/no-empty-interface": "off",
+
+        // Definitions make use of `object` to model rather unusual constraints
+        "@typescript-eslint/ban-types": "off"
+      }
+    },
+
+    // === Compiler Definition rules (extends TypeScript rules) ================
+
+    {
+      files: [
+        "./dist/*.d.ts"
+      ],
+      rules: {
+        // Our definitions are complicated, and all attempts to describe them
+        // as modules have failed so far. As such, we re-export namespaces.
+        "@typescript-eslint/no-namespace": "off",
+        "@typescript-eslint/triple-slash-reference": "off"
+      }
+    },
+
+    // === Test rules (extends TypeScript rules) ===============================
+
+    {
+      files: [
+        "./tests/compiler/**/*.ts",
+        "./lib/loader/tests/assembly/**/*.ts"
+      ],
+      rules: {
+        // Tests typically include unusual code patterns on purpose. This is
+        // very likely not an extensive list, but covers what's there so far.
+        "no-empty": "off",
+        "no-cond-assign": "off",
+        "no-compare-neg-zero": "off",
+        "no-inner-declarations": "off",
+        "no-constant-condition": "off",
+        "use-isnan": "off",
+        "@typescript-eslint/no-namespace": "off",
+        "@typescript-eslint/no-unused-vars": "off",
+        "@typescript-eslint/no-empty-function": "off",
+        "@typescript-eslint/no-non-null-assertion": "off",
+        "@typescript-eslint/no-extra-semi": "off",
+        "@typescript-eslint/no-inferrable-types": "off",
+        "@typescript-eslint/ban-types": "off",
+        "@typescript-eslint/triple-slash-reference": "off",
+        "@typescript-eslint/ban-ts-comment": "off",
+        "@typescript-eslint/no-extra-non-null-assertion": "off",
+        "@typescript-eslint/no-empty-interface": "off"
+      }
+    },
+  ]
+};
diff --git a/cli/index.d.ts b/cli/index.d.ts
@@ -276,7 +276,10 @@ export abstract class Transform {
   /** Called when parsing is complete, before a program is instantiated from the AST. */
   afterParse?(parser: Parser): void | Promise<void>;
 
-  /** Called after the program is instantiated. */
+  /**
+   * Called after the program is instantiated and before compilation-time validation runs.
+   * This is the last hook where transforms can rewrite preserved AST-only syntax before it is rejected.
+   */
   afterInitialize?(program: Program): void | Promise<void>;
 
   /** Called when compilation is complete, before the module is being validated. */

diff --git a/cli/index.js b/cli/index.js
@@ -718,7 +718,7 @@ export async function main(argv, options) {
     stats.initializeTime += stats.end(begin);
   }
 
-  // Call afterInitialize transform hook
+  // Call afterInitialize transform hook, the last AST rewrite point before compilation-time validation.
   {
     let error = await applyTransform("afterInitialize", program);
     if (error) return prepareResult(error);

diff --git a/eslint.config.js b/eslint.config.js
@@ -25,6 +25,10 @@ export default defineConfig([
 
     // FIXME: Tagged template literal tests with invalid escapes
     "tests/compiler/templateliteral.ts",
+
+    // Decorators on `this` are not allowed typically in TypeScript, but this
+    // fixture exercises that AS-only syntax and is validated by transform tests.
+    "tests/transform/parameter-decorators.ts",
   ]),
 
   js.configs.recommended,

diff --git a/package.json b/package.json
@@ -85,8 +85,8 @@
     "test:browser": "node --enable-source-maps tests/browser",
     "test:asconfig": "cd tests/asconfig && npm run test",
     "test:transform": "npm run test:transform:esm && npm run test:transform:cjs",
-    "test:transform:esm": "node bin/asc tests/compiler/empty --transform ./tests/transform/index.js --noEmit && node bin/asc tests/compiler/empty --transform ./tests/transform/simple.js --noEmit",
-    "test:transform:cjs": "node bin/asc tests/compiler/empty --transform ./tests/transform/cjs/index.js --noEmit && node bin/asc tests/compiler/empty --transform ./tests/transform/cjs/simple.js --noEmit",
+    "test:transform:esm": "node bin/asc tests/compiler/empty --transform ./tests/transform/index.js --noEmit && node bin/asc tests/compiler/empty --transform ./tests/transform/simple.js --noEmit && node bin/asc tests/transform/parameter-decorators.ts --transform ./tests/transform/remove-parameter-decorators.js --noEmit",
+    "test:transform:cjs": "node bin/asc tests/compiler/empty --transform ./tests/transform/cjs/index.js --noEmit && node bin/asc tests/compiler/empty --transform ./tests/transform/cjs/simple.js --noEmit && node bin/asc tests/transform/parameter-decorators.ts --transform ./tests/transform/cjs/remove-parameter-decorators.js --noEmit",
     "test:cli": "node tests/cli/options.js",
     "asbuild": "npm run asbuild:debug && npm run asbuild:release",
     "asbuild:debug": "node bin/asc --config src/asconfig.json --target debug",

diff --git a/src/ast.ts b/src/ast.ts
@@ -181,9 +181,12 @@ export abstract class Node {
     name: IdentifierExpression,
     type: TypeNode,
     initializer: Expression | null,
-    range: Range
+    range: Range,
+    decorators: DecoratorNode[] | null = null
   ): ParameterNode {
-    return new ParameterNode(parameterKind, name, type, initializer, range);
+    let parameter = new ParameterNode(parameterKind, name, type, initializer, range);
-    let parameter = new ParameterNode(parameterKind, name, type, initializer, range);
+    let parameter = new ParameterNode(parameterKind, name, type, initializer, decorators, range);
-    let parameter = new ParameterNode(parameterKind, name, type, initializer, range);
+    let parameter = new ParameterNode(parameterKind, name, type, initializer, decorators, range);
+    parameter.decorators = decorators;
+    return parameter;
   }
 
   // special
@@ -926,6 +929,9 @@ export class FunctionTypeNode extends TypeNode {
   ) {
     super(NodeKind.FunctionType, isNullable, range);
   }
+
+  /** Decorators on an explicit `this` parameter, if any, preserved for transforms. */
+  explicitThisDecorators: DecoratorNode[] | null = null;
 }
 
 /** Represents a type parameter. */
@@ -971,6 +977,8 @@ export class ParameterNode extends Node {
     super(NodeKind.Parameter, range);
   }
 
+  /** Decorators, if any, preserved so transforms can rewrite them before validation. */
+  decorators: DecoratorNode[] | null = null;
   /** Implicit field declaration, if applicable. */
   implicitFieldDeclaration: FieldDeclaration | null = null;
   /** Common flags indicating specific traits. */

diff --git a/src/compiler.ts b/src/compiler.ts
@@ -536,6 +536,8 @@ export class Compiler extends DiagnosticEmitter {
 
     // initialize lookup maps, built-ins, imports, exports, etc.
     this.program.initialize();
+    // Reject any parameter decorators that transforms left on the AST.
+    this.program.validateParameterDecorators();
 
 
     // Binaryen treats all function references as being leaked to the outside world when