exclude dead branches

src/Module.js

@@ -7,6 +7,8 @@ import { basename, extname } from './utils/path.js';
 import getLocation from './utils/getLocation.js';
 import makeLegalIdentifier from './utils/makeLegalIdentifier.js';
 import SOURCEMAPPING_URL from './utils/sourceMappingURL.js';
+import { isFalsy, isTruthy } from './ast/conditions.js';
+import { emptyBlockStatement } from './ast/create.js';
 
 class SyntheticDefaultDeclaration {
 	constructor ( node, statement, name ) {
@@ -446,6 +448,17 @@ export default class Module {
 
 		walk( ast, {
 			enter: node => {
+				// eliminate dead branches early
+				if ( node.type === 'IfStatement' ) {
+					if ( isFalsy( node.test ) ) {
+						this.magicString.overwrite( node.consequent.start, node.consequent.end, '{}' );
+						node.consequent = emptyBlockStatement( node.consequent.start, node.consequent.end );
+					} else if ( node.alternate && isTruthy( node.test ) ) {
+						this.magicString.overwrite( node.alternate.start, node.alternate.end, '{}' );
+						node.alternate = emptyBlockStatement( node.alternate.start, node.alternate.end );
+					}
+				}
+
 				this.magicString.addSourcemapLocation( node.start );
 				this.magicString.addSourcemapLocation( node.end );
 			}

src/ast/conditions.js

@@ -0,0 +1,65 @@
+function isEqualTest ( node ) {
+	return node.type === 'BinaryExpression' && ( node.operator === '===' || node.operator === '==' );
+}
+
+function isNotEqualTest ( node ) {
+	return node.type === 'BinaryExpression' && ( node.operator === '!==' || node.operator === '!=' );
+}
+
+function nodesAreEqual ( a, b ) {
+	if ( a.type !== b.type ) return false;
+	if ( a.type === 'Literal' ) return a.value === b.value;
+	if ( a.type === 'Identifier' ) return a.name === b.name;
+
+	return false;
+}
+
+function nodesAreNotEqual ( a, b ) {
+	if ( a.type !== b.type ) return false;
+	if ( a.type === 'Literal' ) return a.value != b.value;
+	if ( a.type === 'Identifier' ) return a.name != b.name;
+
+	return false;
+}
+
+export function isTruthy ( node ) {
+	if ( node.type === 'Literal' && node.value ) return true;
+	if ( node.type === 'ParenthesizedExpression' ) return isTruthy( node.expression );
+
+	if ( isEqualTest( node ) ) return nodesAreEqual( node.left, node.right );
+	if ( isNotEqualTest( node ) ) return nodesAreNotEqual( node.left, node.right );
+
+	if ( node.type === 'UnaryExpression' ) {
+		if ( node.operator === '!' ) return isFalsy( node.argument );
+		return false;
+	}
+
+	if ( node.type === 'LogicalExpression' ) {
+		if ( node.operator === '&&' ) return isTruthy( node.left ) && isTruthy( node.right );
+		if ( node.operator === '||' ) return isTruthy( node.left ) || isTruthy( node.right );
+		return false;
+	}
+
+	return false;
+}
+
+export function isFalsy ( node ) {
+	if ( node.type === 'Literal' && !node.value ) return true;
+	if ( node.type === 'ParenthesizedExpression' ) return isFalsy( node.expression );
+
+	if ( isEqualTest( node ) ) return nodesAreNotEqual( node.left, node.right );
+	if ( isNotEqualTest( node ) ) return nodesAreEqual( node.left, node.right );
+
+	if ( node.type === 'UnaryExpression' ) {
+		if ( node.operator === '!' ) return isTruthy( node.argument );
+		return false;
+	}
+
+	if ( node.type === 'LogicalExpression' ) {
+		if ( node.operator === '&&' ) return isFalsy( node.left ) || isFalsy( node.right );
+		if ( node.operator === '||' ) return isFalsy( node.left ) && isFalsy( node.right );
+		return false;
+	}
+
+	return false;
+}

src/ast/create.js

@@ -0,0 +1,7 @@
+export function emptyBlockStatement ( start, end ) {
+	return {
+		start, end,
+		type: 'BlockStatement',
+		body: []
+	};
+}

test/function/skips-dead-branches-b/_config.js

@@ -0,0 +1,8 @@
+var assert = require( 'assert' );
+
+module.exports = {
+	description: 'skips a dead branch (b)',
+	code: function ( code ) {
+		assert.equal( code.indexOf( 'function foo' ), -1, code );
+	}
+}

test/function/skips-dead-branches-b/main.js

@@ -0,0 +1,10 @@
+function foo () {
+	console.log( 'this should be excluded' );
+}
+
+function bar () {
+	console.log( 'this should be included' );
+}
+
+if ( true ) bar();
+else foo();

test/function/skips-dead-branches-c/_config.js

@@ -0,0 +1,8 @@
+var assert = require( 'assert' );
+
+module.exports = {
+	description: 'skips a dead branch (c)',
+	code: function ( code ) {
+		assert.equal( code.indexOf( 'function foo' ), -1, code );
+	}
+}

test/function/skips-dead-branches-c/main.js

@@ -0,0 +1,10 @@
+function foo () {
+	console.log( 'this should be excluded' );
+}
+
+function bar () {
+	console.log( 'this should be included' );
+}
+
+if ( !true ) foo();
+bar();

test/function/skips-dead-branches-d/_config.js

@@ -0,0 +1,8 @@
+var assert = require( 'assert' );
+
+module.exports = {
+	description: 'skips a dead branch (c)',
+	code: function ( code ) {
+		assert.equal( code.indexOf( 'function foo' ), -1, code );
+	}
+}

test/function/skips-dead-branches-d/main.js

@@ -0,0 +1,10 @@
+function foo () {
+	console.log( 'this should be excluded' );
+}
+
+function bar () {
+	console.log( 'this should be included' );
+}
+
+if ( 'development' === 'production' ) foo();
+bar();

test/function/skips-dead-branches-e/_config.js

@@ -0,0 +1,8 @@
+var assert = require( 'assert' );
+
+module.exports = {
+	description: 'skips a dead branch (c)',
+	code: function ( code ) {
+		assert.equal( code.indexOf( 'function foo' ), -1, code );
+	}
+}

test/function/skips-dead-branches-e/main.js

@@ -0,0 +1,10 @@
+function foo () {
+	console.log( 'this should be excluded' );
+}
+
+function bar () {
+	console.log( 'this should be included' );
+}
+
+if ( 'production' !== 'production' ) foo();
+bar();

test/function/skips-dead-branches/_config.js

@@ -0,0 +1,8 @@
+var assert = require( 'assert' );
+
+module.exports = {
+	description: 'skips a dead branch',
+	code: function ( code ) {
+		assert.equal( code.indexOf( 'function foo' ), -1, code );
+	}
+}

test/function/skips-dead-branches/main.js

@@ -0,0 +1,10 @@
+function foo () {
+	console.log( 'this should be excluded' );
+}
+
+function bar () {
+	console.log( 'this should be included' );
+}
+
+if ( false ) foo();
+else bar();

test/test.js

@@ -161,6 +161,8 @@ describe( 'rollup', function () {
 							code = result.code;
 						}
 
+						if ( config.code ) config.code( code );
+
 						var module = {
 							exports: {}
 						};