From 23c23ae9267ca0cb0a42ebed2bd996c63145eb89 Mon Sep 17 00:00:00 2001
From: Gary King <garyking@gmail.com>
Date: Mon, 9 Mar 2020 15:33:15 -0400
Subject: [PATCH] feat(eslint-plugin): as-const

---
 .../docs/rules/consistent-type-assertions.md  |  2 ++
 .../src/rules/consistent-type-assertions.ts   | 20 +++++++++++++++++--
 .../rules/consistent-type-assertions.test.ts  | 18 +++++++++++++++--
 3 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/packages/eslint-plugin/docs/rules/consistent-type-assertions.md b/packages/eslint-plugin/docs/rules/consistent-type-assertions.md
index cdf0fb7656cb..43e67b755b52 100644
--- a/packages/eslint-plugin/docs/rules/consistent-type-assertions.md
+++ b/packages/eslint-plugin/docs/rules/consistent-type-assertions.md
@@ -8,6 +8,8 @@ Type assertions are also commonly referred as "type casting" in TypeScript (even
 
 In addition to ensuring that type assertions are written in a consistent way, this rule also helps make your codebase more type-safe.
 
+`const` assertions, [introduced in TypeScript 3.4](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-4.html#const-assertions), is always allowed by this rule. Examples of it include `let x = "hello" as const;` and `let x = <const>"hello";`.
+
 ## Options
 
 ```ts
diff --git a/packages/eslint-plugin/src/rules/consistent-type-assertions.ts b/packages/eslint-plugin/src/rules/consistent-type-assertions.ts
index ae54149d0a74..0620822f83c9 100644
--- a/packages/eslint-plugin/src/rules/consistent-type-assertions.ts
+++ b/packages/eslint-plugin/src/rules/consistent-type-assertions.ts
@@ -75,10 +75,27 @@ export default util.createRule<Options, MessageIds>({
   create(context, [options]) {
     const sourceCode = context.getSourceCode();
 
+    function isConst(node: TSESTree.TypeNode): boolean {
+      if (node.type !== AST_NODE_TYPES.TSTypeReference) {
+        return false;
+      }
+
+      return (
+        node.typeName.type === AST_NODE_TYPES.Identifier &&
+        node.typeName.name === 'const'
+      );
+    }
+
     function reportIncorrectAssertionType(
       node: TSESTree.TSTypeAssertion | TSESTree.TSAsExpression,
     ): void {
+      // If this node is `as const`, then don't report an error.
+      if (isConst(node.typeAnnotation)) {
+        return;
+      }
+
       const messageId = options.assertionStyle;
+
       context.report({
         node,
         messageId,
@@ -97,8 +114,7 @@ export default util.createRule<Options, MessageIds>({
         case AST_NODE_TYPES.TSTypeReference:
           return (
             // Ignore `as const` and `<const>`
-            (node.typeName.type === AST_NODE_TYPES.Identifier &&
-              node.typeName.name !== 'const') ||
+            !isConst(node) ||
             // Allow qualified names which have dots between identifiers, `Foo.Bar`
             node.typeName.type === AST_NODE_TYPES.TSQualifiedName
           );
diff --git a/packages/eslint-plugin/tests/rules/consistent-type-assertions.test.ts b/packages/eslint-plugin/tests/rules/consistent-type-assertions.test.ts
index f885d503091f..010bb894ad7a 100644
--- a/packages/eslint-plugin/tests/rules/consistent-type-assertions.test.ts
+++ b/packages/eslint-plugin/tests/rules/consistent-type-assertions.test.ts
@@ -9,14 +9,12 @@ const ANGLE_BRACKET_TESTS = `
 const x = <Foo>new Generic<int>();
 const x = <A>b;
 const x = <readonly number[]>[1];
-const x = <const>[1];
 const x = <a | b>('string');
 `;
 const AS_TESTS = `
 const x = new Generic<int>() as Foo;
 const x = b as A;
 const x = [1] as readonly number[];
-const x = [1] as const;
 const x = ('string') as a | b;
 `;
 const OBJECT_LITERAL_AS_CASTS = `
@@ -98,6 +96,22 @@ ruleTester.run('consistent-type-assertions', rule, {
         },
       ],
     }),
+    {
+      code: 'const x = <const>[1];',
+      options: [
+        {
+          assertionStyle: 'never',
+        },
+      ],
+    },
+    {
+      code: 'const x = [1] as const;',
+      options: [
+        {
+          assertionStyle: 'never',
+        },
+      ],
+    },
   ],
   invalid: [
     ...batchedSingleLineTests({

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