From 4d1e71fdd5aea2f2af271b5dcae284573754a12f Mon Sep 17 00:00:00 2001
From: Rich Harris <richard.a.harris@gmail.com>
Date: Sun, 29 Apr 2018 17:08:24 -0400
Subject: [PATCH] sparse array patterns

---
 src/parse/read/context.ts                     |  9 +++++++--
 src/utils/unpackDestructuring.ts              |  2 ++
 .../_config.js                                | 20 +++++++++++++++++++
 .../main.html                                 |  3 +++
 4 files changed, 32 insertions(+), 2 deletions(-)
 create mode 100644 test/runtime/samples/each-block-destructured-array-sparse/_config.js
 create mode 100644 test/runtime/samples/each-block-destructured-array-sparse/main.html

diff --git a/src/parse/read/context.ts b/src/parse/read/context.ts
index feb31d150e..3ab18695bd 100644
--- a/src/parse/read/context.ts
+++ b/src/parse/read/context.ts
@@ -46,8 +46,13 @@ export default function readContext(parser: Parser) {
 
 		do {
 			parser.allowWhitespace();
-			context.elements.push(readContext(parser));
-			parser.allowWhitespace();
+
+			if (parser.template[parser.index] === ',') {
+				context.elements.push(null);
+			} else {
+				context.elements.push(readContext(parser));
+				parser.allowWhitespace();
+			}
 		} while (parser.eat(','));
 
 		errorOnAssignmentPattern(parser);
diff --git a/src/utils/unpackDestructuring.ts b/src/utils/unpackDestructuring.ts
index 2c48d5c762..749d26b34b 100644
--- a/src/utils/unpackDestructuring.ts
+++ b/src/utils/unpackDestructuring.ts
@@ -3,6 +3,8 @@ export default function unpackDestructuring(
 	node: Node,
 	tail: string
 ) {
+	if (!node) return;
+
 	if (node.type === 'Identifier') {
 		contexts.push({
 			key: node,
diff --git a/test/runtime/samples/each-block-destructured-array-sparse/_config.js b/test/runtime/samples/each-block-destructured-array-sparse/_config.js
new file mode 100644
index 0000000000..f504cfe377
--- /dev/null
+++ b/test/runtime/samples/each-block-destructured-array-sparse/_config.js
@@ -0,0 +1,20 @@
+export default {
+	data: {
+		animalPawsEntries: [
+			['raccoon', 'hands'],
+			['eagle', 'wings']
+		]
+	},
+
+	html: `
+		<p>hands</p>
+		<p>wings</p>
+	`,
+
+	test ( assert, component, target ) {
+		component.set({ animalPawsEntries: [['foo', 'bar']] });
+		assert.htmlEqual( target.innerHTML, `
+			<p>bar</p>
+		`);
+	},
+};
diff --git a/test/runtime/samples/each-block-destructured-array-sparse/main.html b/test/runtime/samples/each-block-destructured-array-sparse/main.html
new file mode 100644
index 0000000000..7ca8e403d8
--- /dev/null
+++ b/test/runtime/samples/each-block-destructured-array-sparse/main.html
@@ -0,0 +1,3 @@
+{#each animalPawsEntries as [, pawType]}
+	<p>{pawType}</p>
+{/each}