Remove conditional expressions if they have no effect.

- if the test evaluation produces an effect, keep the whole expression
- if the test evaluation is unkown but has no effect, remove the expression unless any branch has an effect
- if the test evaluation is known, replace the expression with the resulting branch (as before)

src/ast/nodes/ConditionalExpression.js

@@ -27,6 +27,15 @@ export default class ConditionalExpression extends Node {
 		return testValue ? this.consequent.getValue() : this.alternate.getValue();
 	}
 
+	hasEffects ( options ) {
+		return (
+			this.included
+			|| this.test.hasEffects( options )
+			|| (this.testValue === UNKNOWN_VALUE && (this.consequent.hasEffects( options ) || this.alternate.hasEffects( options )))
+			|| (this.testValue ? this.consequent.hasEffects( options ) : this.alternate.hasEffects( options ))
+		);
+	}
+
 	render ( code, es ) {
 		if ( !this.module.bundle.treeshake ) {
 			super.render( code, es );
@@ -37,22 +46,16 @@ export default class ConditionalExpression extends Node {
 				super.render( code, es );
 			}
 
-			else if ( this.testValue ) {
-				code.remove( this.start, this.consequent.start );
-				code.remove( this.consequent.end, this.end );
-				if ( this.consequent.type === 'SequenceExpression' ) {
-					code.prependRight( this.consequent.start, '(' );
-					code.appendLeft( this.consequent.end, ')' );
-				}
-				this.consequent.render( code, es );
-			} else {
-				code.remove( this.start, this.alternate.start );
-				code.remove( this.alternate.end, this.end );
-				if ( this.alternate.type === 'SequenceExpression' ) {
-					code.prependRight( this.alternate.start, '(' );
-					code.appendLeft( this.alternate.end, ')' );
+			else {
+				const branchToRetain = this.testValue ? this.consequent : this.alternate;
+
+				code.remove( this.start, branchToRetain.start );
+				code.remove( branchToRetain.end, this.end );
+				if ( branchToRetain.type === 'SequenceExpression' ) {
+					code.prependRight( branchToRetain.start, '(' );
+					code.appendLeft( branchToRetain.end, ')' );
 				}
-				this.alternate.render( code, es );
+				branchToRetain.render( code, es );
 			}
 		}
 	}

test/form/samples/conditional-expression/_config.js

@@ -0,0 +1,3 @@
+module.exports = {
+	description: 'only retain branches with side-effects'
+};

test/form/samples/conditional-expression/_expected/amd.js

@@ -0,0 +1,16 @@
+define(function () { 'use strict';
+
+	// side-effect in condition
+	var a = foo() ? 1 : 2;
+
+	var unknownValue = bar();
+
+	// unknown branch with side-effect
+	var c = unknownValue ? foo() : 2;
+	var d = unknownValue ? 1 : foo();
+
+	// known side-effect
+	var h = foo();
+	var i = foo();
+
+});

test/form/samples/conditional-expression/_expected/cjs.js

@@ -0,0 +1,14 @@
+'use strict';
+
+// side-effect in condition
+var a = foo() ? 1 : 2;
+
+var unknownValue = bar();
+
+// unknown branch with side-effect
+var c = unknownValue ? foo() : 2;
+var d = unknownValue ? 1 : foo();
+
+// known side-effect
+var h = foo();
+var i = foo();

test/form/samples/conditional-expression/_expected/es.js

@@ -0,0 +1,12 @@
+// side-effect in condition
+var a = foo() ? 1 : 2;
+
+var unknownValue = bar();
+
+// unknown branch with side-effect
+var c = unknownValue ? foo() : 2;
+var d = unknownValue ? 1 : foo();
+
+// known side-effect
+var h = foo();
+var i = foo();

test/form/samples/conditional-expression/_expected/iife.js

@@ -0,0 +1,17 @@
+(function () {
+	'use strict';
+
+	// side-effect in condition
+	var a = foo() ? 1 : 2;
+
+	var unknownValue = bar();
+
+	// unknown branch with side-effect
+	var c = unknownValue ? foo() : 2;
+	var d = unknownValue ? 1 : foo();
+
+	// known side-effect
+	var h = foo();
+	var i = foo();
+
+}());

test/form/samples/conditional-expression/_expected/umd.js

@@ -0,0 +1,20 @@
+(function (global, factory) {
+	typeof exports === 'object' && typeof module !== 'undefined' ? factory() :
+	typeof define === 'function' && define.amd ? define(factory) :
+	(factory());
+}(this, (function () { 'use strict';
+
+	// side-effect in condition
+	var a = foo() ? 1 : 2;
+
+	var unknownValue = bar();
+
+	// unknown branch with side-effect
+	var c = unknownValue ? foo() : 2;
+	var d = unknownValue ? 1 : foo();
+
+	// known side-effect
+	var h = foo();
+	var i = foo();
+
+})));

test/form/samples/conditional-expression/main.js

@@ -0,0 +1,21 @@
+// side-effect in condition
+var a = foo() ? 1 : 2;
+
+var unknownValue = bar();
+
+// unknown branch without side-effects
+var b = unknownValue ? 1 : 2;
+
+// unknown branch with side-effect
+var c = unknownValue ? foo() : 2;
+var d = unknownValue ? 1 : foo();
+
+// no side-effects
+var e = true ? 1 : foo();
+var f = false ? foo() : 2;
+var g = true ? 1 : 2;
+
+// known side-effect
+var h = true ? foo() : 2;
+var i = false ? 1 : foo();
+