warn on assignment to const

src/compiler/compile/Component.ts

@@ -788,6 +788,33 @@ export default class Component {
 					scope = map.get(node);
 				}
 
+                let deep = false;
+                let names: string[] | null;
+
+                if (node.type === 'AssignmentExpression') {
+						deep = node.left.type === 'MemberExpression';
+						names = deep
+							? [get_object(node.left).name]
+							: extract_names(node.left);
+                } else if (node.type === 'UpdateExpression') {
+                    deep = node.argument.type === 'MemberExpression';
+                    const { name } = get_object(node.argument);
+                    names = [name];
+				}
+
+				
+				if (names) {
+					names.forEach(name => {
+						const variable = component.var_lookup.get(name);
+						if (variable && variable.writable === false && !deep) {
+							component.warn(node as any, {
+								code: 'assignment-to-const',
+								message: 'You are assigning to a const'
+							});
+						}
+					});
+				}
+
 				if (node.type === 'ImportDeclaration') {
 					component.extract_imports(node);
 					// TODO: to use actual remove

src/compiler/compile/nodes/shared/Expression.ts

@@ -62,7 +62,7 @@ export default class Expression {
 
 		// discover dependencies, but don't change the code yet
 		walk(info, {
-			enter(node: any, parent: any, key: string) {
+			enter(node: Node, parent: any, key: string) {
 				// don't manipulate shorthand props twice
 				if (key === 'key' && parent.shorthand) return;
 				// don't manipulate `import.meta`, `new.target`
@@ -126,6 +126,7 @@ export default class Expression {
 						deep = node.left.type === 'MemberExpression';
 						names = extract_names(deep ? get_object(node.left) : node.left);
 					} else if (node.type === 'UpdateExpression') {
+                        deep = node.argument.type === 'MemberExpression';
 						names = extract_names(get_object(node.argument));
 					}
 				}
@@ -146,8 +147,18 @@ export default class Expression {
 						} else {
 							component.add_reference(node, name);
 
-							const variable = component.var_lookup.get(name);
+                            const variable = component.var_lookup.get(name);
-							if (variable) variable[deep ? 'mutated' : 'reassigned'] = true;
+
+							if (variable) {
+								variable[deep ? 'mutated' : 'reassigned'] = true;
+
+								if (!deep && variable.writable === false) {
+									component.warn(node as any, {
+										code: 'assignment-to-const',
+										message: 'You are assigning to a const'
+									});
+								}
+							}
 						}
 					});
 				}

test/validator/samples/assignment-to-const/input.svelte

@@ -0,0 +1,34 @@
+<script>
+	const immutable1 = false;
+	const immutable2 = 0;
+
+	// these ones should not warn
+	const obj1 = { prop: true };
+	const obj2 = { prop: 0 };
+
+	function func1() {
+		immutable1 = true
+	}
+
+	function func2() {
+		immutable2++;
+	}
+
+	function func3() {
+		obj1.prop = false;
+	}
+
+	function func4() {
+		obj2.prop++;
+	}
+</script>
+
+<button on:click={func1}>click</button>
+<button on:click={func2}>click</button>
+<button on:click={func3}>click</button>
+<button on:click={func4}>click</button>
+
+<button on:click={() => immutable2 = true}>click</button>
+<button on:click={() => immutable2++}>click</button>
+<button on:click={() => obj1.prop = false}>click</button>
+<button on:click={() => obj2.prop++}>click</button>

test/validator/samples/assignment-to-const/warnings.json

@@ -0,0 +1,62 @@
+[
+	{
+		"code": "assignment-to-const",
+		"message": "You are assigning to a const",
+		"start": {
+			"line": 31,
+			"column": 24,
+			"character": 512
+		},
+		"end": {
+			"line": 31,
+			"column": 41,
+			"character": 529
+		},
+		"pos": 512
+	},
+	{
+		"code": "assignment-to-const",
+		"message": "You are assigning to a const",
+		"start": {
+			"line": 32,
+			"column": 24,
+			"character": 570
+		},
+		"end": {
+			"line": 32,
+			"column": 36,
+			"character": 582
+		},
+		"pos": 570
+	},
+	{
+		"code": "assignment-to-const",
+		"message": "You are assigning to a const",
+		"start": {
+			"line": 10,
+			"column": 2,
+			"character": 171
+		},
+		"end": {
+			"line": 10,
+			"column": 19,
+			"character": 188
+		},
+		"pos": 171
+	},
+	{
+		"code": "assignment-to-const",
+		"message": "You are assigning to a const",
+		"start": {
+			"line": 14,
+			"column": 2,
+			"character": 215
+		},
+		"end": {
+			"line": 14,
+			"column": 14,
+			"character": 227
+		},
+		"pos": 215
+	}
+]