diff --git a/src/ast/nodes/ForInStatement.js b/src/ast/nodes/ForInStatement.js
new file mode 100644
index 0000000..002dd0a
--- /dev/null
+++ b/src/ast/nodes/ForInStatement.js
@@ -0,0 +1,15 @@
+import Node from '../Node.js';
+import { STRING } from '../values.js';
+
+export default class ForInStatement extends Node {
+	initialise ( scope ) {
+		super.initialise( scope );
+
+		// special case
+		if ( this.left.type === 'VariableDeclaration' ) {
+			for ( const proxy of this.left.declarations[0].proxies.values() ) {
+				proxy.possibleValues.add( STRING );
+			}
+		}
+	}
+}
diff --git a/src/ast/nodes/ForOfStatement.js b/src/ast/nodes/ForOfStatement.js
new file mode 100644
index 0000000..201d491
--- /dev/null
+++ b/src/ast/nodes/ForOfStatement.js
@@ -0,0 +1,15 @@
+import Node from '../Node.js';
+import { UNKNOWN } from '../values.js';
+
+export default class ForOfStatement extends Node {
+	initialise ( scope ) {
+		super.initialise( scope );
+
+		// special case
+		if ( this.left.type === 'VariableDeclaration' ) {
+			for ( const proxy of this.left.declarations[0].proxies.values() ) {
+				proxy.possibleValues.add( UNKNOWN );
+			}
+		}
+	}
+}
diff --git a/src/ast/nodes/index.js b/src/ast/nodes/index.js
index c276a34..3cca1ad 100644
--- a/src/ast/nodes/index.js
+++ b/src/ast/nodes/index.js
@@ -11,6 +11,8 @@ import ExportAllDeclaration from './ExportAllDeclaration.js';
 import ExportDefaultDeclaration from './ExportDefaultDeclaration.js';
 import ExportNamedDeclaration from './ExportNamedDeclaration.js';
 import ExpressionStatement from './ExpressionStatement.js';
+import ForInStatement from './ForInStatement.js';
+import ForOfStatement from './ForOfStatement.js';
 import FunctionDeclaration from './FunctionDeclaration.js';
 import FunctionExpression from './FunctionExpression.js';
 import Identifier from './Identifier.js';
@@ -43,6 +45,8 @@ export default {
 	ExportDefaultDeclaration,
 	ExportNamedDeclaration,
 	ExpressionStatement,
+	ForInStatement,
+	ForOfStatement,
 	FunctionDeclaration,
 	FunctionExpression,
 	Identifier,
diff --git a/test/form/effect-in-for-of-loop/_config.js b/test/form/effect-in-for-of-loop/_config.js
new file mode 100644
index 0000000..79abe74
--- /dev/null
+++ b/test/form/effect-in-for-of-loop/_config.js
@@ -0,0 +1,3 @@
+module.exports = {
+	description: 'includes effects in for-of loop (#870)'
+}
diff --git a/test/form/effect-in-for-of-loop/_expected/amd.js b/test/form/effect-in-for-of-loop/_expected/amd.js
new file mode 100644
index 0000000..57223a0
--- /dev/null
+++ b/test/form/effect-in-for-of-loop/_expected/amd.js
@@ -0,0 +1,19 @@
+define(function () { 'use strict';
+
+	const items = [{}, {}, {}];
+
+	function x () {
+		for ( const item of items.children ) {
+			item.foo = 'bar';
+		}
+	}
+
+	x();
+
+	assert.deepEqual( items, [
+		{ foo: 'bar' },
+		{ foo: 'bar' },
+		{ foo: 'bar' }
+	]);
+
+});
diff --git a/test/form/effect-in-for-of-loop/_expected/cjs.js b/test/form/effect-in-for-of-loop/_expected/cjs.js
new file mode 100644
index 0000000..fd94a9d
--- /dev/null
+++ b/test/form/effect-in-for-of-loop/_expected/cjs.js
@@ -0,0 +1,17 @@
+'use strict';
+
+const items = [{}, {}, {}];
+
+function x () {
+	for ( const item of items.children ) {
+		item.foo = 'bar';
+	}
+}
+
+x();
+
+assert.deepEqual( items, [
+	{ foo: 'bar' },
+	{ foo: 'bar' },
+	{ foo: 'bar' }
+]);
diff --git a/test/form/effect-in-for-of-loop/_expected/es.js b/test/form/effect-in-for-of-loop/_expected/es.js
new file mode 100644
index 0000000..cabffc4
--- /dev/null
+++ b/test/form/effect-in-for-of-loop/_expected/es.js
@@ -0,0 +1,15 @@
+const items = [{}, {}, {}];
+
+function x () {
+	for ( const item of items.children ) {
+		item.foo = 'bar';
+	}
+}
+
+x();
+
+assert.deepEqual( items, [
+	{ foo: 'bar' },
+	{ foo: 'bar' },
+	{ foo: 'bar' }
+]);
diff --git a/test/form/effect-in-for-of-loop/_expected/iife.js b/test/form/effect-in-for-of-loop/_expected/iife.js
new file mode 100644
index 0000000..96e91ea
--- /dev/null
+++ b/test/form/effect-in-for-of-loop/_expected/iife.js
@@ -0,0 +1,20 @@
+(function () {
+	'use strict';
+
+	const items = [{}, {}, {}];
+
+	function x () {
+		for ( const item of items.children ) {
+			item.foo = 'bar';
+		}
+	}
+
+	x();
+
+	assert.deepEqual( items, [
+		{ foo: 'bar' },
+		{ foo: 'bar' },
+		{ foo: 'bar' }
+	]);
+
+}());
diff --git a/test/form/effect-in-for-of-loop/_expected/umd.js b/test/form/effect-in-for-of-loop/_expected/umd.js
new file mode 100644
index 0000000..83a6d30
--- /dev/null
+++ b/test/form/effect-in-for-of-loop/_expected/umd.js
@@ -0,0 +1,23 @@
+(function (global, factory) {
+	typeof exports === 'object' && typeof module !== 'undefined' ? factory() :
+	typeof define === 'function' && define.amd ? define(factory) :
+	(factory());
+}(this, (function () { 'use strict';
+
+	const items = [{}, {}, {}];
+
+	function x () {
+		for ( const item of items.children ) {
+			item.foo = 'bar';
+		}
+	}
+
+	x();
+
+	assert.deepEqual( items, [
+		{ foo: 'bar' },
+		{ foo: 'bar' },
+		{ foo: 'bar' }
+	]);
+
+})));
diff --git a/test/form/effect-in-for-of-loop/main.js b/test/form/effect-in-for-of-loop/main.js
new file mode 100644
index 0000000..401dcf2
--- /dev/null
+++ b/test/form/effect-in-for-of-loop/main.js
@@ -0,0 +1,23 @@
+const items = [{}, {}, {}];
+
+function x () {
+	for ( const item of items.children ) {
+		item.foo = 'bar';
+	}
+}
+
+x();
+
+function y () {
+	for ( const item of items.children ) {
+		// do nothing
+	}
+}
+
+y();
+
+assert.deepEqual( items, [
+	{ foo: 'bar' },
+	{ foo: 'bar' },
+	{ foo: 'bar' }
+]);