optimise certain $$invalidate calls

src/compile/render-dom/index.ts

@@ -188,6 +188,12 @@ export default function dom(
 
 						code.overwrite(node.start, node.end, dirty.map(n => component.invalidate(n)).join('; '));
 					} else {
+						const single = (
+							node.left.type === 'Identifier' &&
+							parent.type === 'ExpressionStatement' &&
+							node.left.name[0] !== '$'
+						);
+
 						names.forEach(name => {
 							const owner = scope.find_owner(name);
 							if (owner && owner !== component.instance_scope) return;
@@ -195,7 +201,13 @@ export default function dom(
 							const variable = component.var_lookup.get(name);
 							if (variable && (variable.hoistable || variable.global || variable.module)) return;
 
-							pending_assignments.add(name);
+							if (single) {
+								code.prependRight(node.start, `$$invalidate('${name}', `);
+								code.appendLeft(node.end, `)`);
+							} else {
+								pending_assignments.add(name);
+							}
+
 							component.has_reactive_assignments = true;
 						});
 					}

test/js/samples/dev-warning-missing-data-computed/expected.js

@@ -70,7 +70,7 @@ function instance($$self, $$props, $$invalidate) {
 	};
 
 	$$self.$$.update = ($$dirty = { foo: 1 }) => {
-		if ($$dirty.foo) { bar = foo * 2; $$invalidate('bar', bar); }
+		if ($$dirty.foo) { $$invalidate('bar', bar = foo * 2); }
 	};
 
 	return { foo, bar };

test/js/samples/instrumentation-script-if-no-block/expected.js

@@ -61,7 +61,7 @@ function instance($$self, $$props, $$invalidate) {
 	let x = 0;
 
 	function foo() {
-		if (true) { x += 1; $$invalidate('x', x); }
+		if (true) $$invalidate('x', x += 1);
 	}
 
 	return { x, foo };

test/js/samples/reactive-values-non-topologically-ordered/expected.js

@@ -28,8 +28,8 @@ function instance($$self, $$props, $$invalidate) {
 	};
 
 	$$self.$$.update = ($$dirty = { x: 1, b: 1 }) => {
-		if ($$dirty.x) { b = x; $$invalidate('b', b); }
+		if ($$dirty.x) { $$invalidate('b', b = x); }
-		if ($$dirty.b) { a = b; $$invalidate('a', a); }
+		if ($$dirty.b) { $$invalidate('a', a = b); }
 	};
 
 	return { x };

test/js/samples/reactive-values-non-writable-dependencies/expected.js

@@ -27,7 +27,7 @@ function instance($$self, $$props, $$invalidate) {
 	let max;
 
 	$$self.$$.update = ($$dirty = { a: 1, b: 1 }) => {
-		if ($$dirty.a || $$dirty.b) { max = Math.max(a, b); $$invalidate('max', max); }
+		if ($$dirty.a || $$dirty.b) { $$invalidate('max', max = Math.max(a, b)); }
 	};
 
 	return {};