From b78b2dd99f0bfa149874901d19f087c7adaef628 Mon Sep 17 00:00:00 2001
From: Lukas Taegert-Atkinson <lukas.taegert-atkinson@tngtech.com>
Date: Sat, 19 Jul 2025 06:49:38 +0200
Subject: [PATCH] Make const reassignments only a warning

---
 src/ast/nodes/AssignmentExpression.ts         | 11 +++++---
 .../samples/error-const-reassign/_config.js   | 21 ---------------
 .../samples/warning-const-reassign/_config.js | 27 +++++++++++++++++++
 .../main.js                                   |  0
 4 files changed, 35 insertions(+), 24 deletions(-)
 delete mode 100644 test/function/samples/error-const-reassign/_config.js
 create mode 100644 test/function/samples/warning-const-reassign/_config.js
 rename test/function/samples/{error-const-reassign => warning-const-reassign}/main.js (100%)

diff --git a/src/ast/nodes/AssignmentExpression.ts b/src/ast/nodes/AssignmentExpression.ts
index f6e395699ab..baee66bb64e 100644
--- a/src/ast/nodes/AssignmentExpression.ts
+++ b/src/ast/nodes/AssignmentExpression.ts
@@ -1,5 +1,6 @@
 import type MagicString from 'magic-string';
 import { BLANK } from '../../utils/blank';
+import { LOGLEVEL_WARN } from '../../utils/logging';
 import { logConstVariableReassignError } from '../../utils/logs';
 import {
 	findFirstOccurrenceOutsideComment,
@@ -48,13 +49,15 @@ export default class AssignmentExpression extends NodeBase {
 		| '??=';
 	declare right: ExpressionNode;
 	declare type: NodeType.tAssignmentExpression;
+	private isConstReassignment = false;
 
 	hasEffects(context: HasEffectsContext): boolean {
-		const { deoptimized, left, operator, right } = this;
+		const { deoptimized, isConstReassignment, left, operator, right } = this;
 		if (!deoptimized) this.applyDeoptimizations();
 		// MemberExpressions do not access the property before assignments if the
 		// operator is '='.
 		return (
+			isConstReassignment ||
 			right.hasEffects(context) ||
 			left.hasEffectsAsAssignmentTarget(context, operator !== '=') ||
 			this.left.hasEffectsWhenDestructuring?.(context, EMPTY_PATH, right)
@@ -70,12 +73,13 @@ export default class AssignmentExpression extends NodeBase {
 	}
 
 	include(context: InclusionContext, includeChildrenRecursively: IncludeChildren): void {
-		const { deoptimized, left, right, operator } = this;
+		const { deoptimized, isConstReassignment, left, right, operator } = this;
 		if (!deoptimized) this.applyDeoptimizations();
 		if (!this.included) this.includeNode(context);
 		const hasEffectsContext = createHasEffectsContext();
 		if (
 			includeChildrenRecursively ||
+			isConstReassignment ||
 			operator !== '=' ||
 			left.included ||
 			left.hasEffectsAsAssignmentTarget(hasEffectsContext, false) ||
@@ -97,7 +101,8 @@ export default class AssignmentExpression extends NodeBase {
 		if (this.left instanceof Identifier) {
 			const variable = this.scope.variables.get(this.left.name);
 			if (variable?.kind === 'const') {
-				this.scope.context.error(logConstVariableReassignError(), this.left.start);
+				this.isConstReassignment = true;
+				this.scope.context.log(LOGLEVEL_WARN, logConstVariableReassignError(), this.left.start);
 			}
 		}
 		this.left.setAssignedValue(this.right);
diff --git a/test/function/samples/error-const-reassign/_config.js b/test/function/samples/error-const-reassign/_config.js
deleted file mode 100644
index 6763e27b00d..00000000000
--- a/test/function/samples/error-const-reassign/_config.js
+++ /dev/null
@@ -1,21 +0,0 @@
-const path = require('node:path');
-const MAIN_ID = path.resolve(__dirname, 'main.js');
-
-module.exports = defineTest({
-	description: 'Cannot reassign a variable declared with `const`',
-	error: {
-		code: 'CONST_REASSIGN',
-		frame: `
-			1: const foo = 1;\n2: foo = 2;\n   ^
-		`,
-		id: MAIN_ID,
-		loc: {
-			column: 0,
-			file: MAIN_ID,
-			line: 2
-		},
-		message: 'main.js (2:0): Cannot reassign a variable declared with `const`',
-		pos: 15,
-		watchFiles: [MAIN_ID]
-	}
-});
diff --git a/test/function/samples/warning-const-reassign/_config.js b/test/function/samples/warning-const-reassign/_config.js
new file mode 100644
index 00000000000..468399c1356
--- /dev/null
+++ b/test/function/samples/warning-const-reassign/_config.js
@@ -0,0 +1,27 @@
+const path = require('node:path');
+const assert = require('node:assert/strict');
+const MAIN_ID = path.resolve(__dirname, 'main.js');
+
+module.exports = defineTest({
+	solo: true,
+	description: 'Cannot reassign a variable declared with `const`',
+	warnings: [
+		{
+			code: 'CONST_REASSIGN',
+			frame: `
+		  	1: const foo = 1;\n2: foo = 2;\n   ^
+		  `,
+			id: MAIN_ID,
+			loc: {
+				column: 0,
+				file: MAIN_ID,
+				line: 2
+			},
+			message: 'main.js (2:0): Cannot reassign a variable declared with `const`',
+			pos: 15
+		}
+	],
+	runtimeError(error) {
+		assert.equal(error.name, 'TypeError');
+	}
+});
diff --git a/test/function/samples/error-const-reassign/main.js b/test/function/samples/warning-const-reassign/main.js
similarity index 100%
rename from test/function/samples/error-const-reassign/main.js
rename to test/function/samples/warning-const-reassign/main.js