diff --git a/src/Bundle.js b/src/Bundle.js
index c4c5edf..8bc5826 100644
--- a/src/Bundle.js
+++ b/src/Bundle.js
@@ -73,14 +73,9 @@ export default class Bundle {
 					declaration.use();
 				});
 
-				let settled = false;
-				while ( !settled ) {
-					settled = true;
-
-					this.modules.forEach( module => {
-						if ( module.markAllSideEffects() ) settled = false;
-					});
-				}
+				this.modules.forEach( module => {
+					module.markAllSideEffects();
+				});
 
 				this.orderedModules = this.sort();
 				this.deconflict();
diff --git a/src/Declaration.js b/src/Declaration.js
index caefdca..7fb7c86 100644
--- a/src/Declaration.js
+++ b/src/Declaration.js
@@ -1,10 +1,5 @@
-import { walk } from 'estree-walker';
-import { keys } from './utils/object';
-
-const modifierNodes = {
-	AssignmentExpression: 'left',
-	UpdateExpression: 'argument'
-};
+import { blank, keys } from './utils/object.js';
+import testForSideEffects from './utils/testForSideEffects.js';
 
 export default class Declaration {
 	constructor ( node ) {
@@ -22,6 +17,7 @@ export default class Declaration {
 		this.name = null;
 
 		this.isReassigned = false;
+		this.mutations = [];
 		this.aliases = [];
 	}
 
@@ -34,54 +30,195 @@ export default class Declaration {
 		this.name = reference.name; // TODO handle differences of opinion
 
 		if ( reference.isReassignment ) this.isReassigned = true;
+		if ( reference.isMutation && !~this.mutations.indexOf( reference.statement ) ) {
+			this.mutations.push( reference.statement );
+		}
 	}
 
-	mutates () {
-		// returns a list of things this function mutates when it gets called
-		if ( !this._mutates ) {
-			let mutatedNames = {};
+	hasSideEffect () {
+		return testForSideEffects( this.functionBody, this.statement.scope, this.statement );
+	}
 
-			const statement = this.statement;
-			let scope = statement.scope;
+	render ( es6 ) {
+		if ( es6 ) return this.name;
+		if ( !this.isReassigned || !this.isExported ) return this.name;
 
-			const addNode = node => {
-				while ( node.type === 'MemberExpression' ) node = node.object;
-				if ( node.type === 'Identifier' ) mutatedNames[ node.name ] = true;
-			};
+		return `exports.${this.name}`;
+	}
 
-			walk( this.functionBody, {
-				enter ( node ) {
-					if ( node._scope ) scope = node._scope;
+	use () {
+		this.isUsed = true;
+		if ( this.statement ) this.statement.mark();
 
-					if ( node.type in modifierNodes ) {
-						addNode( node[ modifierNodes[ node.type ] ] );
-					} else if ( node.type === 'CallExpression' ) {
-						addNode( node.callee );
-					}
-				},
+		this.aliases.forEach( alias => alias.use() );
+	}
+}
 
-				leave ( node ) {
-					if ( node._scope ) scope = scope.parent;
-				}
-			});
+export class SyntheticDefaultDeclaration {
+	constructor ( node, statement, name ) {
+		this.node = node;
+		this.statement = statement;
+		this.name = name;
 
-			this._mutates = keys( mutatedNames );
-		}
+		this.original = null;
+		this.isExported = false;
+		this.aliases = [];
+	}
 
-		return this._mutates;
+	addAlias ( declaration ) {
+		this.aliases.push( declaration );
 	}
 
-	render ( es6 ) {
-		if ( es6 ) return this.name;
-		if ( !this.isReassigned || !this.isExported ) return this.name;
+	addReference ( reference ) {
+		// Don't change the name to `default`; it's not a valid identifier name.
+		if ( reference.name === 'default' ) return;
 
-		return `exports.${this.name}`;
+		reference.declaration = this;
+		this.name = reference.name;
+	}
+
+	bind ( declaration ) {
+		this.original = declaration;
+	}
+
+	hasSideEffect () {
+		if ( this.original ) {
+			return this.original.hasSideEffect();
+		}
+		
+		if ( /FunctionExpression/.test( this.node.declaration.type ) ) {
+			return testForSideEffects( this.node.declaration.body, this.statement.scope, this.statement );
+		}
+	}
+
+	render () {
+		return !this.original || this.original.isReassigned ?
+			this.name :
+			this.original.render();
 	}
 
 	use () {
 		this.isUsed = true;
-		if ( this.statement ) this.statement.mark();
+		this.statement.mark();
+
+		if ( this.original ) this.original.use();
 
 		this.aliases.forEach( alias => alias.use() );
 	}
 }
+
+export class SyntheticNamespaceDeclaration {
+	constructor ( module ) {
+		this.module = module;
+		this.name = null;
+
+		this.needsNamespaceBlock = false;
+		this.aliases = [];
+
+		this.originals = blank();
+		module.getExports().forEach( name => {
+			this.originals[ name ] = module.traceExport( name );
+		});
+	}
+
+	addAlias ( declaration ) {
+		this.aliases.push( declaration );
+	}
+
+	addReference ( reference ) {
+		// if we have e.g. `foo.bar`, we can optimise
+		// the reference by pointing directly to `bar`
+		if ( reference.parts.length ) {
+			reference.name = reference.parts.shift();
+
+			reference.end += reference.name.length + 1; // TODO this is brittle
+
+			const original = this.originals[ reference.name ];
+
+			// throw with an informative error message if the reference doesn't exist.
+			if ( !original ) {
+				this.module.bundle.onwarn( `Export '${reference.name}' is not defined by '${this.module.id}'` );
+				reference.isUndefined = true;
+				return;
+			}
+
+			original.addReference( reference );
+			return;
+		}
+
+		// otherwise we're accessing the namespace directly,
+		// which means we need to mark all of this module's
+		// exports and render a namespace block in the bundle
+		if ( !this.needsNamespaceBlock ) {
+			this.needsNamespaceBlock = true;
+			this.module.bundle.internalNamespaces.push( this );
+		}
+
+		reference.declaration = this;
+		this.name = reference.name;
+	}
+
+	renderBlock ( indentString ) {
+		const members = keys( this.originals ).map( name => {
+			const original = this.originals[ name ];
+
+			if ( original.isReassigned ) {
+				return `${indentString}get ${name} () { return ${original.render()}; }`;
+			}
+
+			return `${indentString}${name}: ${original.render()}`;
+		});
+
+		return `var ${this.render()} = Object.freeze({\n${members.join( ',\n' )}\n});\n\n`;
+	}
+
+	render () {
+		return this.name;
+	}
+
+	use () {
+		keys( this.originals ).forEach( name => {
+			this.originals[ name ].use();
+		});
+
+		this.aliases.forEach( alias => alias.use() );
+	}
+}
+
+export class ExternalDeclaration {
+	constructor ( module, name ) {
+		this.module = module;
+		this.name = name;
+		this.isExternal = true;
+	}
+
+	addAlias () {
+		// noop
+	}
+
+	addReference ( reference ) {
+		reference.declaration = this;
+
+		if ( this.name === 'default' || this.name === '*' ) {
+			this.module.suggestName( reference.name );
+		}
+	}
+
+	render ( es6 ) {
+		if ( this.name === '*' ) {
+			return this.module.name;
+		}
+
+		if ( this.name === 'default' ) {
+			return !es6 && this.module.exportsNames ?
+				`${this.module.name}__default` :
+				this.module.name;
+		}
+
+		return es6 ? this.name : `${this.module.name}.${this.name}`;
+	}
+
+	use () {
+		// noop?
+	}
+}
diff --git a/src/ExternalModule.js b/src/ExternalModule.js
index 0e6a3ac..3113a5d 100644
--- a/src/ExternalModule.js
+++ b/src/ExternalModule.js
@@ -1,43 +1,6 @@
 import { blank } from './utils/object.js';
 import makeLegalIdentifier from './utils/makeLegalIdentifier.js';
-
-class ExternalDeclaration {
-	constructor ( module, name ) {
-		this.module = module;
-		this.name = name;
-		this.isExternal = true;
-	}
-
-	addAlias () {
-		// noop
-	}
-
-	addReference ( reference ) {
-		reference.declaration = this;
-
-		if ( this.name === 'default' || this.name === '*' ) {
-			this.module.suggestName( reference.name );
-		}
-	}
-
-	render ( es6 ) {
-		if ( this.name === '*' ) {
-			return this.module.name;
-		}
-
-		if ( this.name === 'default' ) {
-			return !es6 && this.module.exportsNames ?
-				`${this.module.name}__default` :
-				this.module.name;
-		}
-
-		return es6 ? this.name : `${this.module.name}.${this.name}`;
-	}
-
-	use () {
-		// noop?
-	}
-}
+import { ExternalDeclaration } from './Declaration.js';
 
 export default class ExternalModule {
 	constructor ( id ) {
diff --git a/src/Module.js b/src/Module.js
index 863a246..1e5a97b 100644
--- a/src/Module.js
+++ b/src/Module.js
@@ -7,131 +7,7 @@ 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';
-
-class SyntheticDefaultDeclaration {
-	constructor ( node, statement, name ) {
-		this.node = node;
-		this.statement = statement;
-		this.name = name;
-
-		this.original = null;
-		this.isExported = false;
-		this.aliases = [];
-	}
-
-	addAlias ( declaration ) {
-		this.aliases.push( declaration );
-	}
-
-	addReference ( reference ) {
-		// Don't change the name to `default`; it's not a valid identifier name.
-		if ( reference.name === 'default' ) return;
-
-		reference.declaration = this;
-		this.name = reference.name;
-	}
-
-	bind ( declaration ) {
-		this.original = declaration;
-	}
-
-	mutates () {
-		return this.original.mutates();
-	}
-
-	render () {
-		return !this.original || this.original.isReassigned ?
-			this.name :
-			this.original.render();
-	}
-
-	use () {
-		this.isUsed = true;
-		this.statement.mark();
-
-		if ( this.original ) this.original.use();
-
-		this.aliases.forEach( alias => alias.use() );
-	}
-}
-
-class SyntheticNamespaceDeclaration {
-	constructor ( module ) {
-		this.module = module;
-		this.name = null;
-
-		this.needsNamespaceBlock = false;
-		this.aliases = [];
-
-		this.originals = blank();
-		module.getExports().forEach( name => {
-			this.originals[ name ] = module.traceExport( name );
-		});
-	}
-
-	addAlias ( declaration ) {
-		this.aliases.push( declaration );
-	}
-
-	addReference ( reference ) {
-		// if we have e.g. `foo.bar`, we can optimise
-		// the reference by pointing directly to `bar`
-		if ( reference.parts.length ) {
-			reference.name = reference.parts.shift();
-
-			reference.end += reference.name.length + 1; // TODO this is brittle
-
-			const original = this.originals[ reference.name ];
-
-			// throw with an informative error message if the reference doesn't exist.
-			if ( !original ) {
-				this.module.bundle.onwarn( `Export '${reference.name}' is not defined by '${this.module.id}'` );
-				reference.isUndefined = true;
-				return;
-			}
-
-			original.addReference( reference );
-			return;
-		}
-
-		// otherwise we're accessing the namespace directly,
-		// which means we need to mark all of this module's
-		// exports and render a namespace block in the bundle
-		if ( !this.needsNamespaceBlock ) {
-			this.needsNamespaceBlock = true;
-			this.module.bundle.internalNamespaces.push( this );
-		}
-
-		reference.declaration = this;
-		this.name = reference.name;
-	}
-
-	renderBlock ( indentString ) {
-		const members = keys( this.originals ).map( name => {
-			const original = this.originals[ name ];
-
-			if ( original.isReassigned ) {
-				return `${indentString}get ${name} () { return ${original.render()}; }`;
-			}
-
-			return `${indentString}${name}: ${original.render()}`;
-		});
-
-		return `var ${this.render()} = Object.freeze({\n${members.join( ',\n' )}\n});\n\n`;
-	}
-
-	render () {
-		return this.name;
-	}
-
-	use () {
-		keys( this.originals ).forEach( name => {
-			this.originals[ name ].use();
-		});
-
-		this.aliases.forEach( alias => alias.use() );
-	}
-}
+import { SyntheticDefaultDeclaration, SyntheticNamespaceDeclaration } from './Declaration.js';
 
 export default class Module {
 	constructor ({ id, code, originalCode, ast, sourceMapChain, bundle }) {
@@ -411,13 +287,9 @@ export default class Module {
 	}
 
 	markAllSideEffects () {
-		let hasSideEffect = false;
-
 		this.statements.forEach( statement => {
-			if ( statement.markSideEffect() ) hasSideEffect = true;
+			statement.markSideEffect();
 		});
-
-		return hasSideEffect;
 	}
 
 	namespace () {
diff --git a/src/Statement.js b/src/Statement.js
index e0a83b1..31b8bd7 100644
--- a/src/Statement.js
+++ b/src/Statement.js
@@ -1,16 +1,9 @@
 import { walk } from 'estree-walker';
 import Scope from './ast/Scope.js';
 import attachScopes from './ast/attachScopes.js';
+import modifierNodes from './ast/modifierNodes.js';
 import getLocation from './utils/getLocation.js';
-
-const modifierNodes = {
-	AssignmentExpression: 'left',
-	UpdateExpression: 'argument'
-};
-
-function isIife ( node, parent ) {
-	return parent && parent.type === 'CallExpression' && node === parent.callee;
-}
+import testForSideEffects from './utils/testForSideEffects.js';
 
 function isReference ( node, parent ) {
 	if ( node.type === 'MemberExpression' ) {
@@ -35,9 +28,10 @@ function isReference ( node, parent ) {
 }
 
 class Reference {
-	constructor ( node, scope ) {
+	constructor ( node, scope, statement ) {
 		this.node = node;
 		this.scope = scope;
+		this.statement = statement;
 
 		this.declaration = null; // bound later
 
@@ -90,6 +84,7 @@ export default class Statement {
 		});
 
 		// find references
+		const statement = this;
 		let { module, references, scope, stringLiteralRanges } = this;
 		let readDepth = 0;
 
@@ -106,7 +101,7 @@ export default class Statement {
 				}
 
 				if ( node._scope ) scope = node._scope;
-				if ( /Function/.test( node.type ) && !isIife( node, parent ) ) readDepth += 1;
+				if ( /Function/.test( node.type ) ) readDepth += 1;
 
 				// special case – shorthand properties. because node.key === node.value,
 				// we can't differentiate once we've descended into the node
@@ -117,9 +112,10 @@ export default class Statement {
 					return this.skip();
 				}
 
+				const isMutation = parent && parent.type in modifierNodes;
 				let isReassignment;
 
-				if ( parent && parent.type in modifierNodes ) {
+				if ( isMutation ) {
 					let subject = parent[ modifierNodes[ parent.type ] ];
 					let depth = 0;
 
@@ -153,18 +149,19 @@ export default class Statement {
 						scope.parent :
 						scope;
 
-					const reference = new Reference( node, referenceScope );
+					const reference = new Reference( node, referenceScope, statement );
 					references.push( reference );
 
 					reference.isImmediatelyUsed = !readDepth;
 					reference.isReassignment = isReassignment;
+					reference.isMutation = !readDepth && isMutation;
 
 					this.skip(); // don't descend from `foo.bar.baz` into `foo.bar`
 				}
 			},
-			leave ( node, parent ) {
+			leave ( node ) {
 				if ( node._scope ) scope = scope.parent;
-				if ( /Function/.test( node.type ) && !isIife( node, parent ) ) readDepth -= 1;
+				if ( /Function/.test( node.type ) ) readDepth -= 1;
 			}
 		});
 	}
@@ -181,82 +178,10 @@ export default class Statement {
 	markSideEffect () {
 		if ( this.isIncluded ) return;
 
-		const statement = this;
-		let hasSideEffect = false;
-
-		walk( this.node, {
-			enter ( node ) {
-				// Don't descend into (quasi) function declarations – we'll worry about those
-				// if they get called
-				if ( node.type === 'FunctionDeclaration' || node.type === 'VariableDeclarator' && node.init && /FunctionExpression/.test( node.init.type ) ) {
-					return this.skip();
-				}
-
-				// If this is a top-level call expression, or an assignment to a global,
-				// this statement will need to be marked
-				if ( node.type === 'NewExpression' ) {
-					hasSideEffect = true;
-				}
-
-				else if ( node.type === 'CallExpression' ) {
-					if ( node.callee.type === 'Identifier' ) {
-						const declaration = statement.module.trace( node.callee.name );
-
-						if ( !declaration || !declaration.isFunctionDeclaration ) {
-							hasSideEffect = true;
-						}
-
-						else {
-							const mutatedByFunction = declaration.mutates();
-							let i = mutatedByFunction.length;
-							while ( i-- ) {
-								const mutatedDeclaration = statement.module.trace( mutatedByFunction[i] );
-
-								if ( !mutatedDeclaration || mutatedDeclaration.isUsed ) {
-									hasSideEffect = true;
-									break;
-								}
-							}
-
-							// if ( declaration.hasSideEffect() ) {
-							// 	// if calling this function creates side-effects...
-							// 	hasSideEffect = true;
-							// }
-							//
-							// else {
-							// 	// ...or mutates inputs that are included...
-							// 	hasSideEffect = true;
-							// }
-
-							// TODO does function mutate inputs that are needed?
-						}
-					}
-
-					else if ( node.callee.type === 'MemberExpression' ) {
-						// if we're calling e.g. Object.keys(thing), there are no side-effects
-						// TODO
-
-						hasSideEffect = true;
-					}
-				}
-
-				else if ( node.type in modifierNodes ) {
-					let subject = node[ modifierNodes[ node.type ] ];
-					while ( subject.type === 'MemberExpression' ) subject = subject.object;
-
-					const declaration = statement.module.trace( subject.name );
-
-					if ( !declaration || declaration.isExternal || declaration.statement.isIncluded ) {
-						hasSideEffect = true;
-					}
-				}
-
-				if ( hasSideEffect ) this.skip();
-			}
-		});
-
-		if ( hasSideEffect ) statement.mark();
-		return hasSideEffect;
+		if ( testForSideEffects( this.node, this.scope, this ) ) {
+			this.mark();
+			return true;
+		}
 	}
 
 	source () {
diff --git a/src/ast/flatten.js b/src/ast/flatten.js
new file mode 100644
index 0000000..0cecbde
--- /dev/null
+++ b/src/ast/flatten.js
@@ -0,0 +1,16 @@
+export default function flatten ( node ) {
+	let parts = [];
+	while ( node.type === 'MemberExpression' ) {
+		if ( node.computed ) return null;
+		parts.unshift( node.property.name );
+
+		node = node.object;
+	}
+
+	if ( node.type !== 'Identifier' ) return null;
+
+	const name = node.name;
+	parts.unshift( name );
+
+	return { name, keypath: parts.join( '.' ) };
+}
diff --git a/src/ast/modifierNodes.js b/src/ast/modifierNodes.js
new file mode 100644
index 0000000..2ea3f5d
--- /dev/null
+++ b/src/ast/modifierNodes.js
@@ -0,0 +1,4 @@
+export default {
+	AssignmentExpression: 'left',
+	UpdateExpression: 'argument'
+};
diff --git a/src/utils/testForSideEffects.js b/src/utils/testForSideEffects.js
new file mode 100644
index 0000000..1aa4f96
--- /dev/null
+++ b/src/utils/testForSideEffects.js
@@ -0,0 +1,92 @@
+import { walk } from 'estree-walker';
+import modifierNodes from '../ast/modifierNodes.js';
+import flatten from '../ast/flatten';
+
+let pureFunctions = {};
+[
+	// TODO add others to this list
+	'Object.keys'
+].forEach( name => pureFunctions[ name ] = true );
+
+export default function testForSideEffects ( node, scope, statement ) {
+	let hasSideEffect = false;
+
+	walk( node, {
+		enter ( node ) {
+			if ( hasSideEffect ) return this.skip();
+			if ( /Function/.test( node.type ) ) return this.skip();
+
+			if ( node._scope ) scope = node._scope;
+
+			// If this is a top-level call expression, or an assignment to a global,
+			// this statement will need to be marked
+			if ( node.type === 'NewExpression' ) {
+				hasSideEffect = true;
+				return this.skip();
+			}
+
+			if ( node.type === 'CallExpression' ) {
+				if ( node.callee.type === 'Identifier' && !scope.contains( node.callee.name ) ) {
+					const declaration = statement.module.trace( node.callee.name );
+
+					if ( !declaration || declaration.isExternal ) {
+						// we're calling a global or an external function. Assume side-effects
+						hasSideEffect = true;
+						return this.skip();
+					}
+
+					// we're calling a function defined in this bundle
+					if ( declaration.hasSideEffect() ) {
+						hasSideEffect = true;
+						return this.skip();
+					}
+
+					// TODO does function mutate inputs that are needed?
+					return;
+				}
+
+				if ( node.callee.type === 'MemberExpression' ) {
+					const flattened = flatten( node.callee );
+
+					if ( !flattened ) {
+						// is not a keypath like `foo.bar.baz` – could be e.g.
+						// `(a || b).foo()`. Err on the side of caution
+						hasSideEffect = true;
+						return;
+					}
+
+					// if we're calling e.g. Object.keys(thing), there are no side-effects
+					// TODO make pureFunctions configurable
+					const declaration = statement.module.trace( flattened.name );
+					if ( !declaration && pureFunctions[ flattened.keypath ] ) return;
+
+					hasSideEffect = true;
+					return this.skip();
+				}
+
+				// otherwise we're probably dealing with a function expression
+				if ( testForSideEffects( node.callee, scope, statement ) ) {
+					hasSideEffect = true;
+					return this.skip();
+				}
+			}
+
+			if ( node.type in modifierNodes ) {
+				let subject = node[ modifierNodes[ node.type ] ];
+				while ( subject.type === 'MemberExpression' ) subject = subject.object;
+
+				const declaration = statement.module.trace( subject.name );
+
+				if ( !declaration || declaration.isExternal || declaration.statement.isIncluded ) {
+					hasSideEffect = true;
+					return this.skip();
+				}
+			}
+		},
+		leave ( node ) {
+			if ( node._scope ) scope = scope.parent;
+		}
+	});
+
+	return hasSideEffect;
+}
diff --git a/test/form/namespace-optimization-b/_expected/amd.js b/test/form/namespace-optimization-b/_expected/amd.js
index 44fba59..30c5406 100644
--- a/test/form/namespace-optimization-b/_expected/amd.js
+++ b/test/form/namespace-optimization-b/_expected/amd.js
@@ -1,15 +1,19 @@
 define(function () { 'use strict';
 
-  function foo() {
-  };
-
-  function a() {
-    foo();
-    foo();
-    var a;
-    if (a.b) {
-    }
-  }
-  a();
-
-});
+	function foo () {
+		console.log( 'foo' );
+	}
+
+	function a () {
+		foo();
+		foo();
+
+		var a;
+		if ( a.b ) {
+			// empty
+		}
+	}
+
+	a();
+
+});
\ No newline at end of file
diff --git a/test/form/namespace-optimization-b/_expected/cjs.js b/test/form/namespace-optimization-b/_expected/cjs.js
index 34f819f..3ee6869 100644
--- a/test/form/namespace-optimization-b/_expected/cjs.js
+++ b/test/form/namespace-optimization-b/_expected/cjs.js
@@ -1,13 +1,17 @@
 'use strict';
 
-function foo() {
-};
-
-function a() {
-  foo();
-  foo();
-  var a;
-  if (a.b) {
-  }
+function foo () {
+	console.log( 'foo' );
 }
-a();
+
+function a () {
+	foo();
+	foo();
+
+	var a;
+	if ( a.b ) {
+		// empty
+	}
+}
+
+a();
\ No newline at end of file
diff --git a/test/form/namespace-optimization-b/_expected/es6.js b/test/form/namespace-optimization-b/_expected/es6.js
index c1a6d5d..8346d0d 100644
--- a/test/form/namespace-optimization-b/_expected/es6.js
+++ b/test/form/namespace-optimization-b/_expected/es6.js
@@ -1,11 +1,15 @@
-function foo() {
-};
+function foo () {
+	console.log( 'foo' );
+}
+
+function a () {
+	foo();
+	foo();
 
-function a() {
-  foo();
-  foo();
-  var a;
-  if (a.b) {
-  }
+	var a;
+	if ( a.b ) {
+		// empty
+	}
 }
-a();
+
+a();
\ No newline at end of file
diff --git a/test/form/namespace-optimization-b/_expected/iife.js b/test/form/namespace-optimization-b/_expected/iife.js
index 6ea6190..59b2ce7 100644
--- a/test/form/namespace-optimization-b/_expected/iife.js
+++ b/test/form/namespace-optimization-b/_expected/iife.js
@@ -1,15 +1,19 @@
 (function () { 'use strict';
 
-  function foo() {
-  };
-
-  function a() {
-    foo();
-    foo();
-    var a;
-    if (a.b) {
-    }
-  }
-  a();
-
-})();
+	function foo () {
+		console.log( 'foo' );
+	}
+
+	function a () {
+		foo();
+		foo();
+
+		var a;
+		if ( a.b ) {
+			// empty
+		}
+	}
+
+	a();
+
+})();
\ No newline at end of file
diff --git a/test/form/namespace-optimization-b/_expected/umd.js b/test/form/namespace-optimization-b/_expected/umd.js
index 108f27b..c2ad84b 100644
--- a/test/form/namespace-optimization-b/_expected/umd.js
+++ b/test/form/namespace-optimization-b/_expected/umd.js
@@ -1,19 +1,23 @@
 (function (global, factory) {
-  typeof exports === 'object' && typeof module !== 'undefined' ? factory() :
-  typeof define === 'function' && define.amd ? define(factory) :
-  factory();
+	typeof exports === 'object' && typeof module !== 'undefined' ? factory() :
+	typeof define === 'function' && define.amd ? define(factory) :
+	factory();
 }(this, function () { 'use strict';
 
-  function foo() {
-  };
+	function foo () {
+		console.log( 'foo' );
+	}
 
-  function a() {
-    foo();
-    foo();
-    var a;
-    if (a.b) {
-    }
-  }
-  a();
+	function a () {
+		foo();
+		foo();
 
-}));
+		var a;
+		if ( a.b ) {
+			// empty
+		}
+	}
+
+	a();
+
+}));
\ No newline at end of file
diff --git a/test/form/self-contained-bundle/_expected/amd.js b/test/form/self-contained-bundle/_expected/amd.js
index ed04c7c..a29d399 100644
--- a/test/form/self-contained-bundle/_expected/amd.js
+++ b/test/form/self-contained-bundle/_expected/amd.js
@@ -1,7 +1,7 @@
 define(function () { 'use strict';
 
 	function foo () {
-		return bar();
+		console.log( bar() );
 	}
 
 	function bar () {
diff --git a/test/form/self-contained-bundle/_expected/cjs.js b/test/form/self-contained-bundle/_expected/cjs.js
index 8a71fe7..499be1d 100644
--- a/test/form/self-contained-bundle/_expected/cjs.js
+++ b/test/form/self-contained-bundle/_expected/cjs.js
@@ -1,7 +1,7 @@
 'use strict';
 
 function foo () {
-	return bar();
+	console.log( bar() );
 }
 
 function bar () {
diff --git a/test/form/self-contained-bundle/_expected/es6.js b/test/form/self-contained-bundle/_expected/es6.js
index 7d5460c..5588fc7 100644
--- a/test/form/self-contained-bundle/_expected/es6.js
+++ b/test/form/self-contained-bundle/_expected/es6.js
@@ -1,5 +1,5 @@
 function foo () {
-	return bar();
+	console.log( bar() );
 }
 
 function bar () {
diff --git a/test/form/self-contained-bundle/_expected/iife.js b/test/form/self-contained-bundle/_expected/iife.js
index 46f6561..1017054 100644
--- a/test/form/self-contained-bundle/_expected/iife.js
+++ b/test/form/self-contained-bundle/_expected/iife.js
@@ -1,7 +1,7 @@
 (function () { 'use strict';
 
 	function foo () {
-		return bar();
+		console.log( bar() );
 	}
 
 	function bar () {
diff --git a/test/form/self-contained-bundle/_expected/umd.js b/test/form/self-contained-bundle/_expected/umd.js
index 51b72b0..d29ef88 100644
--- a/test/form/self-contained-bundle/_expected/umd.js
+++ b/test/form/self-contained-bundle/_expected/umd.js
@@ -5,7 +5,7 @@
 }(this, function () { 'use strict';
 
 	function foo () {
-		return bar();
+		console.log( bar() );
 	}
 
 	function bar () {
diff --git a/test/form/self-contained-bundle/foo.js b/test/form/self-contained-bundle/foo.js
index 1e911f3..46a9052 100644
--- a/test/form/self-contained-bundle/foo.js
+++ b/test/form/self-contained-bundle/foo.js
@@ -1,5 +1,5 @@
 export default function foo () {
-	return bar();
+	console.log( bar() );
 }
 
 function bar () {
diff --git a/test/form/side-effect-b/_config.js b/test/form/side-effect-b/_config.js
new file mode 100644
index 0000000..9b0c852
--- /dev/null
+++ b/test/form/side-effect-b/_config.js
@@ -0,0 +1,6 @@
+module.exports = {
+	description: 'discards IIFE with no side-effects',
+	options: {
+		moduleName: 'myBundle'
+	}
+};
diff --git a/test/form/side-effect-b/_expected/amd.js b/test/form/side-effect-b/_expected/amd.js
new file mode 100644
index 0000000..70cc3c4
--- /dev/null
+++ b/test/form/side-effect-b/_expected/amd.js
@@ -0,0 +1,7 @@
+define(function () { 'use strict';
+
+	var main = 42;
+
+	return main;
+
+});
\ No newline at end of file
diff --git a/test/form/side-effect-b/_expected/cjs.js b/test/form/side-effect-b/_expected/cjs.js
new file mode 100644
index 0000000..cc51dcb
--- /dev/null
+++ b/test/form/side-effect-b/_expected/cjs.js
@@ -0,0 +1,5 @@
+'use strict';
+
+var main = 42;
+
+module.exports = main;
\ No newline at end of file
diff --git a/test/form/side-effect-b/_expected/es6.js b/test/form/side-effect-b/_expected/es6.js
new file mode 100644
index 0000000..b953a15
--- /dev/null
+++ b/test/form/side-effect-b/_expected/es6.js
@@ -0,0 +1,3 @@
+var main = 42;
+
+export default main;
\ No newline at end of file
diff --git a/test/form/side-effect-b/_expected/iife.js b/test/form/side-effect-b/_expected/iife.js
new file mode 100644
index 0000000..0880329
--- /dev/null
+++ b/test/form/side-effect-b/_expected/iife.js
@@ -0,0 +1,7 @@
+var myBundle = (function () { 'use strict';
+
+	var main = 42;
+
+	return main;
+
+})();
\ No newline at end of file
diff --git a/test/form/side-effect-b/_expected/umd.js b/test/form/side-effect-b/_expected/umd.js
new file mode 100644
index 0000000..7f83fb9
--- /dev/null
+++ b/test/form/side-effect-b/_expected/umd.js
@@ -0,0 +1,11 @@
+(function (global, factory) {
+	typeof exports === 'object' && typeof module !== 'undefined' ? module.exports = factory() :
+	typeof define === 'function' && define.amd ? define(factory) :
+	global.myBundle = factory();
+}(this, function () { 'use strict';
+
+	var main = 42;
+
+	return main;
+
+}));
\ No newline at end of file
diff --git a/test/form/side-effect-b/main.js b/test/form/side-effect-b/main.js
new file mode 100644
index 0000000..c5e65eb
--- /dev/null
+++ b/test/form/side-effect-b/main.js
@@ -0,0 +1,8 @@
+var Unused = (function () {
+	function Unused () {}
+	Unused.prototype = {};
+
+	return Unused;
+}());
+
+export default 42;
diff --git a/test/form/side-effect-c/_config.js b/test/form/side-effect-c/_config.js
new file mode 100644
index 0000000..0d692d4
--- /dev/null
+++ b/test/form/side-effect-c/_config.js
@@ -0,0 +1,6 @@
+module.exports = {
+	description: 'discards function with no side-effects',
+	options: {
+		moduleName: 'myBundle'
+	}
+};
diff --git a/test/form/side-effect-c/_expected/amd.js b/test/form/side-effect-c/_expected/amd.js
new file mode 100644
index 0000000..70cc3c4
--- /dev/null
+++ b/test/form/side-effect-c/_expected/amd.js
@@ -0,0 +1,7 @@
+define(function () { 'use strict';
+
+	var main = 42;
+
+	return main;
+
+});
\ No newline at end of file
diff --git a/test/form/side-effect-c/_expected/cjs.js b/test/form/side-effect-c/_expected/cjs.js
new file mode 100644
index 0000000..cc51dcb
--- /dev/null
+++ b/test/form/side-effect-c/_expected/cjs.js
@@ -0,0 +1,5 @@
+'use strict';
+
+var main = 42;
+
+module.exports = main;
\ No newline at end of file
diff --git a/test/form/side-effect-c/_expected/es6.js b/test/form/side-effect-c/_expected/es6.js
new file mode 100644
index 0000000..b953a15
--- /dev/null
+++ b/test/form/side-effect-c/_expected/es6.js
@@ -0,0 +1,3 @@
+var main = 42;
+
+export default main;
\ No newline at end of file
diff --git a/test/form/side-effect-c/_expected/iife.js b/test/form/side-effect-c/_expected/iife.js
new file mode 100644
index 0000000..0880329
--- /dev/null
+++ b/test/form/side-effect-c/_expected/iife.js
@@ -0,0 +1,7 @@
+var myBundle = (function () { 'use strict';
+
+	var main = 42;
+
+	return main;
+
+})();
\ No newline at end of file
diff --git a/test/form/side-effect-c/_expected/umd.js b/test/form/side-effect-c/_expected/umd.js
new file mode 100644
index 0000000..7f83fb9
--- /dev/null
+++ b/test/form/side-effect-c/_expected/umd.js
@@ -0,0 +1,11 @@
+(function (global, factory) {
+	typeof exports === 'object' && typeof module !== 'undefined' ? module.exports = factory() :
+	typeof define === 'function' && define.amd ? define(factory) :
+	global.myBundle = factory();
+}(this, function () { 'use strict';
+
+	var main = 42;
+
+	return main;
+
+}));
\ No newline at end of file
diff --git a/test/form/side-effect-c/main.js b/test/form/side-effect-c/main.js
new file mode 100644
index 0000000..87ad109
--- /dev/null
+++ b/test/form/side-effect-c/main.js
@@ -0,0 +1,10 @@
+var factory = function () {
+	function Unused () {}
+	Unused.prototype = {};
+
+	return Unused;
+};
+
+var Unused = factory();
+
+export default 42;
diff --git a/test/form/side-effect-d/_config.js b/test/form/side-effect-d/_config.js
new file mode 100644
index 0000000..a8d5bf9
--- /dev/null
+++ b/test/form/side-effect-d/_config.js
@@ -0,0 +1,6 @@
+module.exports = {
+	description: 'excludes functions that are known to be pure',
+	options: {
+		moduleName: 'myBundle'
+	}
+};
diff --git a/test/form/side-effect-d/_expected/amd.js b/test/form/side-effect-d/_expected/amd.js
new file mode 100644
index 0000000..37d2571
--- /dev/null
+++ b/test/form/side-effect-d/_expected/amd.js
@@ -0,0 +1,7 @@
+define(function () { 'use strict';
+
+	var main = 42;
+
+	return main;
+
+});
diff --git a/test/form/side-effect-d/_expected/cjs.js b/test/form/side-effect-d/_expected/cjs.js
new file mode 100644
index 0000000..5a370cd
--- /dev/null
+++ b/test/form/side-effect-d/_expected/cjs.js
@@ -0,0 +1,5 @@
+'use strict';
+
+var main = 42;
+
+module.exports = main;
diff --git a/test/form/side-effect-d/_expected/es6.js b/test/form/side-effect-d/_expected/es6.js
new file mode 100644
index 0000000..d862de8
--- /dev/null
+++ b/test/form/side-effect-d/_expected/es6.js
@@ -0,0 +1,3 @@
+var main = 42;
+
+export default main;
diff --git a/test/form/side-effect-d/_expected/iife.js b/test/form/side-effect-d/_expected/iife.js
new file mode 100644
index 0000000..b7b15ee
--- /dev/null
+++ b/test/form/side-effect-d/_expected/iife.js
@@ -0,0 +1,7 @@
+var myBundle = (function () { 'use strict';
+
+	var main = 42;
+
+	return main;
+
+})();
diff --git a/test/form/side-effect-d/_expected/umd.js b/test/form/side-effect-d/_expected/umd.js
new file mode 100644
index 0000000..b5aa08a
--- /dev/null
+++ b/test/form/side-effect-d/_expected/umd.js
@@ -0,0 +1,11 @@
+(function (global, factory) {
+	typeof exports === 'object' && typeof module !== 'undefined' ? module.exports = factory() :
+	typeof define === 'function' && define.amd ? define(factory) :
+	global.myBundle = factory();
+}(this, function () { 'use strict';
+
+	var main = 42;
+
+	return main;
+
+}));
diff --git a/test/form/side-effect-d/main.js b/test/form/side-effect-d/main.js
new file mode 100644
index 0000000..c53646d
--- /dev/null
+++ b/test/form/side-effect-d/main.js
@@ -0,0 +1,4 @@
+var obj = { foo: 1, bar: 2 };
+var keys = Object.keys( obj );
+
+export default 42;
diff --git a/test/form/unused-side-effect/_config.js b/test/form/side-effect/_config.js
similarity index 100%
rename from test/form/unused-side-effect/_config.js
rename to test/form/side-effect/_config.js
diff --git a/test/form/unused-side-effect/_expected/amd.js b/test/form/side-effect/_expected/amd.js
similarity index 100%
rename from test/form/unused-side-effect/_expected/amd.js
rename to test/form/side-effect/_expected/amd.js
diff --git a/test/form/unused-side-effect/_expected/cjs.js b/test/form/side-effect/_expected/cjs.js
similarity index 100%
rename from test/form/unused-side-effect/_expected/cjs.js
rename to test/form/side-effect/_expected/cjs.js
diff --git a/test/form/unused-side-effect/_expected/es6.js b/test/form/side-effect/_expected/es6.js
similarity index 100%
rename from test/form/unused-side-effect/_expected/es6.js
rename to test/form/side-effect/_expected/es6.js
diff --git a/test/form/unused-side-effect/_expected/iife.js b/test/form/side-effect/_expected/iife.js
similarity index 100%
rename from test/form/unused-side-effect/_expected/iife.js
rename to test/form/side-effect/_expected/iife.js
diff --git a/test/form/unused-side-effect/_expected/umd.js b/test/form/side-effect/_expected/umd.js
similarity index 100%
rename from test/form/unused-side-effect/_expected/umd.js
rename to test/form/side-effect/_expected/umd.js
diff --git a/test/form/unused-side-effect/foo.js b/test/form/side-effect/foo.js
similarity index 100%
rename from test/form/unused-side-effect/foo.js
rename to test/form/side-effect/foo.js
diff --git a/test/form/unused-side-effect/main.js b/test/form/side-effect/main.js
similarity index 100%
rename from test/form/unused-side-effect/main.js
rename to test/form/side-effect/main.js
diff --git a/test/form/unmodified-default-exports-function-argument/_expected/amd.js b/test/form/unmodified-default-exports-function-argument/_expected/amd.js
index c1053c2..be41871 100644
--- a/test/form/unmodified-default-exports-function-argument/_expected/amd.js
+++ b/test/form/unmodified-default-exports-function-argument/_expected/amd.js
@@ -11,4 +11,6 @@ define(function () { 'use strict';
 	var answer = foo();
 	var somethingElse = bar();
 
+	console.log( answer );
+
 });
diff --git a/test/form/unmodified-default-exports-function-argument/_expected/cjs.js b/test/form/unmodified-default-exports-function-argument/_expected/cjs.js
index 18131b8..362f677 100644
--- a/test/form/unmodified-default-exports-function-argument/_expected/cjs.js
+++ b/test/form/unmodified-default-exports-function-argument/_expected/cjs.js
@@ -10,3 +10,5 @@ function bar () {
 
 var answer = foo();
 var somethingElse = bar();
+
+console.log( answer );
diff --git a/test/form/unmodified-default-exports-function-argument/_expected/es6.js b/test/form/unmodified-default-exports-function-argument/_expected/es6.js
index 6bde701..76d6f0c 100644
--- a/test/form/unmodified-default-exports-function-argument/_expected/es6.js
+++ b/test/form/unmodified-default-exports-function-argument/_expected/es6.js
@@ -8,3 +8,5 @@ function bar () {
 
 var answer = foo();
 var somethingElse = bar();
+
+console.log( answer );
diff --git a/test/form/unmodified-default-exports-function-argument/_expected/iife.js b/test/form/unmodified-default-exports-function-argument/_expected/iife.js
index 8d5d37f..9fb882d 100644
--- a/test/form/unmodified-default-exports-function-argument/_expected/iife.js
+++ b/test/form/unmodified-default-exports-function-argument/_expected/iife.js
@@ -11,4 +11,6 @@
 	var answer = foo();
 	var somethingElse = bar();
 
+	console.log( answer );
+
 })();
diff --git a/test/form/unmodified-default-exports-function-argument/_expected/umd.js b/test/form/unmodified-default-exports-function-argument/_expected/umd.js
index 5ae4301..2a4c64c 100644
--- a/test/form/unmodified-default-exports-function-argument/_expected/umd.js
+++ b/test/form/unmodified-default-exports-function-argument/_expected/umd.js
@@ -15,4 +15,6 @@
 	var answer = foo();
 	var somethingElse = bar();
 
+	console.log( answer );
+
 }));
diff --git a/test/form/unmodified-default-exports-function-argument/main.js b/test/form/unmodified-default-exports-function-argument/main.js
index 1cc9361..efe5d3f 100644
--- a/test/form/unmodified-default-exports-function-argument/main.js
+++ b/test/form/unmodified-default-exports-function-argument/main.js
@@ -2,3 +2,5 @@ import foo, { bar } from './foo';
 
 var answer = foo();
 var somethingElse = bar();
+
+console.log( answer );