Fix additional let/var init bugs

docs/999-big-list-of-options.md

@@ -1431,22 +1431,6 @@ Default: `false`
 If a variable is assigned a value in its declaration and is never reassigned, Rollup sometimes assumes the value to be constant. This is not true if the variable is declared with `var`, however, as those variables can be accessed before their declaration where they will evaluate to `undefined`.
 Choosing `true` will make sure Rollup does not make (wrong) assumptions about the value of such variables. Note though that this can have a noticeable negative impact on tree-shaking results.
 
-```js
-// input
-if (Math.random() < 0.5) var x = true;
-if (!x) {
-  console.log('effect');
-}
-
-// no output with treeshake.correctVarValueBeforeDeclaration === false
-
-// output with treeshake.correctVarValueBeforeDeclaration === true
-if (Math.random() < 0.5) var x = true;
-if (!x) {
-  console.log('effect');
-}
-```
-
 **treeshake.moduleSideEffects**<br>
 Type: `boolean | "no-external" | string[] | (id: string, external: boolean) => boolean`<br>
 CLI: `--treeshake.moduleSideEffects`/`--no-treeshake.moduleSideEffects`/`--treeshake.moduleSideEffects no-external`<br>

src/ast/nodes/Identifier.ts

@@ -245,6 +245,19 @@ export default class Identifier extends NodeBase implements PatternNode {
 			return (this.isTDZAccess = false);
 		}
 
+		let decl_id;
+		if (
+			this.variable.declarations &&
+			this.variable.declarations.length === 1 &&
+			(decl_id = this.variable.declarations[0] as any) &&
+			this.start < decl_id.start &&
+			closestParentFunctionOrProgram(this) === closestParentFunctionOrProgram(decl_id)
+		) {
+			// a variable accessed before its declaration
+			// in the same function or at top level of module
+			return (this.isTDZAccess = true);
+		}
+
 		if (!this.variable.initReached) {
 			// Either a const/let TDZ violation or
 			// var use before declaration was encountered.
@@ -254,3 +267,11 @@ export default class Identifier extends NodeBase implements PatternNode {
 		return (this.isTDZAccess = false);
 	}
 }
+
+function closestParentFunctionOrProgram(node: any): any {
+	while (node && !/^Program|Function/.test(node.type)) {
+		node = node.parent;
+	}
+	// one of: ArrowFunctionExpression, FunctionDeclaration, FunctionExpression or Program
+	return node;
+}

src/ast/scopes/TrackingScope.ts

@@ -14,6 +14,6 @@ export default class TrackingScope extends BlockScope {
 		isHoisted: boolean
 	): LocalVariable {
 		this.hoistedDeclarations.push(identifier);
-		return this.parent.addDeclaration(identifier, context, init, isHoisted);
+		return super.addDeclaration(identifier, context, init, isHoisted);
 	}
 }

test/cli/samples/treeshake-preset-override/_expected.js

@@ -11,3 +11,17 @@ try {
 } catch {}
 
 unknownGlobal;
+
+let flag = true;
+
+function test() {
+	if (flag) var x = true;
+	if (x) {
+		return;
+	}
+	console.log('effect');
+}
+
+test();
+flag = false;
+test();

test/form/samples/tdz-common/_expected.js

@@ -41,3 +41,16 @@ class cls {}
 	console.log("C" );
 	console.log("D" );
 })();
+
+(function let_tdz() {
+	let flag = false;
+	function foo() {
+		if (flag) {
+			value; // TDZ
+		}
+		let value;
+	}
+	foo();
+	flag = true;
+	foo();
+})();

test/form/samples/tdz-common/main.js

@@ -55,3 +55,26 @@ class cls {}
 	console.log(C ? "C" : "!C");
 	console.log(D ? "D" : "!D");
 })();
+
+(function let_tdz() {
+	let flag = false;
+	function foo() {
+		if (flag) {
+			value; // TDZ
+		}
+		let value;
+	}
+	foo();
+	flag = true;
+	foo();
+})();
+
+// should be dropped
+(function() {
+	function foo() {
+		const access = () => value;
+		let value;
+		access();
+	};
+	foo();
+})();

test/form/samples/treeshake-presets/preset-with-override/_expected.js

@@ -1,3 +1,17 @@
 console.log('main');
 
 unknownGlobal;
+
+let flag = true;
+
+function test() {
+	if (flag) var x = true;
+	if (x) {
+		return;
+	}
+	console.log('effect');
+}
+
+test();
+flag = false;
+test();

test/form/samples/treeshake-presets/recommended/_expected.js

@@ -11,3 +11,17 @@ console.log('main');
 try {
 	const noeffect = 1;
 } catch {}
+
+let flag = true;
+
+function test() {
+	if (flag) var x = true;
+	if (x) {
+		return;
+	}
+	console.log('effect');
+}
+
+test();
+flag = false;
+test();

test/form/samples/treeshake-presets/smallest/_expected.js

@@ -1 +1,15 @@
 console.log('main');
+
+let flag = true;
+
+function test() {
+	if (flag) var x = true;
+	if (x) {
+		return;
+	}
+	console.log('effect');
+}
+
+test();
+flag = false;
+test();

test/form/samples/treeshake-presets/true/_expected.js

@@ -13,3 +13,17 @@ try {
 } catch {}
 
 unknownGlobal;
+
+let flag = true;
+
+function test() {
+	if (flag) var x = true;
+	if (x) {
+		return;
+	}
+	console.log('effect');
+}
+
+test();
+flag = false;
+test();

test/function/samples/use-var-before-decl/main.js

@@ -41,4 +41,21 @@ log(function() {
 	}
 })();
 
-assert.strictEqual(results.join(" "), "PASS1 PASS2 PASS3 PASS4 PASS5 PASS6 OFF ON");
+(function () {
+	var flag = false;
+
+	function foo() {
+		if (flag) {
+			if (!value) log(flag + "1");
+		}
+		var value = true;
+		log(flag + "2");
+	}
+
+	foo();
+
+	flag = true;
+	foo();
+})();
+
+assert.strictEqual(results.join(" "), "PASS1 PASS2 PASS3 PASS4 PASS5 PASS6 OFF ON false2 true1 true2");