From d249c7eb34a8cbe20fff21108ac3db73af2301c9 Mon Sep 17 00:00:00 2001
From: Rich-Harris <richard.a.harris@gmail.com>
Date: Tue, 27 Dec 2016 16:37:01 -0500
Subject: [PATCH] preserve var declarations in dead branches

---
 package.json                                  |  1 -
 src/ast/nodes/IfStatement.js                  | 43 +++++++++++++++++++
 .../_config.js                                |  3 ++
 .../main.js                                   |  5 +++
 4 files changed, 51 insertions(+), 1 deletion(-)
 create mode 100644 test/function/preserves-var-declarations-in-dead-branches/_config.js
 create mode 100644 test/function/preserves-var-declarations-in-dead-branches/main.js

diff --git a/package.json b/package.json
index 71d71ba..1b90828 100644
--- a/package.json
+++ b/package.json
@@ -51,7 +51,6 @@
     "console-group": "^0.3.1",
     "eslint": "^2.13.1",
     "eslint-plugin-import": "^2.2.0",
-    "estree-walker": "^0.2.1",
     "is-reference": "^1.0.0",
     "istanbul": "^0.4.3",
     "magic-string": "^0.15.2",
diff --git a/src/ast/nodes/IfStatement.js b/src/ast/nodes/IfStatement.js
index fe83a78..87c8792 100644
--- a/src/ast/nodes/IfStatement.js
+++ b/src/ast/nodes/IfStatement.js
@@ -1,4 +1,5 @@
 import Statement from './shared/Statement.js';
+import extractNames from '../utils/extractNames.js';
 import { UNKNOWN } from '../values.js';
 
 // Statement types which may contain if-statements as direct children.
@@ -11,9 +12,34 @@ const statementsWithIfStatements = new Set([
 	'WhileStatement'
 ]);
 
+function handleVarDeclarations ( node, scope ) {
+	const hoistedVars = [];
+
+	function visit ( node ) {
+		if ( node.type === 'VariableDeclaration' && node.kind === 'var' ) {
+			node.initialise( scope );
+
+			node.declarations.forEach( declarator => {
+				extractNames( declarator.id ).forEach( name => {
+					if ( !~hoistedVars.indexOf( name ) ) hoistedVars.push( name );
+				});
+			});
+		}
+
+		else if ( !/Function/.test( node.type ) ) {
+			node.eachChild( visit );
+		}
+	}
+
+	visit( node );
+
+	return hoistedVars;
+}
+
 // TODO DRY this out
 export default class IfStatement extends Statement {
 	initialise ( scope ) {
+		this.scope = scope;
 		this.testValue = this.test.getValue();
 
 		if ( this.module.bundle.treeshake ) {
@@ -23,11 +49,15 @@ export default class IfStatement extends Statement {
 
 			else if ( this.testValue ) {
 				this.consequent.initialise( scope );
+
+				if ( this.alternate ) this.hoistedVars = handleVarDeclarations( this.alternate, scope );
 				this.alternate = null;
 			}
 
 			else {
 				if ( this.alternate ) this.alternate.initialise( scope );
+
+				this.hoistedVars = handleVarDeclarations( this.consequent, scope );
 				this.consequent = null;
 			}
 		}
@@ -49,6 +79,19 @@ export default class IfStatement extends Statement {
 				// TODO if no block-scoped declarations, remove enclosing
 				// curlies and dedent block (if there is a block)
 
+				if ( this.hoistedVars ) {
+					const names = this.hoistedVars
+						.map( name => {
+							const declaration = this.scope.findDeclaration( name );
+							return declaration.activated ? declaration.getName() : null;
+						})
+						.filter( Boolean );
+
+					if ( names.length > 0 ) {
+						code.insertLeft( this.start, `var ${names.join( ', ' )};\n\n` );
+					}
+				}
+
 				if ( this.testValue ) {
 					code.remove( this.start, this.consequent.start );
 					code.remove( this.consequent.end, this.end );
diff --git a/test/function/preserves-var-declarations-in-dead-branches/_config.js b/test/function/preserves-var-declarations-in-dead-branches/_config.js
new file mode 100644
index 0000000..e0f687e
--- /dev/null
+++ b/test/function/preserves-var-declarations-in-dead-branches/_config.js
@@ -0,0 +1,3 @@
+module.exports = {
+	description: 'preserves var declarations in dead branches (#977)'
+};
diff --git a/test/function/preserves-var-declarations-in-dead-branches/main.js b/test/function/preserves-var-declarations-in-dead-branches/main.js
new file mode 100644
index 0000000..523b8cc
--- /dev/null
+++ b/test/function/preserves-var-declarations-in-dead-branches/main.js
@@ -0,0 +1,5 @@
+if ( false ) {
+	var foo;
+}
+
+assert.equal( foo, undefined );