use maps for keyed each block lookups - fixes #2430

src/compile/render-dom/wrappers/EachBlock.ts

@@ -294,7 +294,7 @@ export default class EachBlockWrapper extends Wrapper {
 		const lookup = block.get_unique_name(`${this.var}_lookup`);
 
 		block.add_variable(iterations, '[]');
-		block.add_variable(lookup, `@blank_object()`);
+		block.add_variable(lookup, `new Map()`);
 
 		if (this.fragment.nodes[0].is_dom_node()) {
 			this.block.first = this.fragment.nodes[0].var;
@@ -314,7 +314,7 @@ export default class EachBlockWrapper extends Wrapper {
 			for (var #i = 0; #i < ${this.vars.each_block_value}.${length}; #i += 1) {
 				let child_ctx = ${this.vars.get_each_context}(ctx, ${this.vars.each_block_value}, #i);
 				let key = ${get_key}(child_ctx);
-				${iterations}[#i] = ${lookup}[key] = ${create_each_block}(key, child_ctx);
+				${lookup}.set(key, ${iterations}[#i] = ${create_each_block}(key, child_ctx));
 			}
 		`);
 

src/internal/keyed-each.js

@@ -2,7 +2,7 @@ import { on_outro } from './transitions.js';
 
 export function destroy_block(block, lookup) {
 	block.d(1);
-	lookup[block.key] = null;
+	lookup.delete(block.key);
 }
 
 export function outro_and_destroy_block(block, lookup) {
@@ -27,14 +27,14 @@ export function update_keyed_each(old_blocks, changed, get_key, dynamic, ctx, li
 	while (i--) old_indexes[old_blocks[i].key] = i;
 
 	const new_blocks = [];
-	const new_lookup = {};
+	const new_lookup = new Map();
-	const deltas = {};
+	const deltas = new Map();
 
 	i = n;
 	while (i--) {
 		const child_ctx = get_context(ctx, list, i);
 		const key = get_key(child_ctx);
-		let block = lookup[key];
+		let block = lookup.get(key);
 
 		if (!block) {
 			block = create_each_block(key, child_ctx);
@@ -43,18 +43,18 @@ export function update_keyed_each(old_blocks, changed, get_key, dynamic, ctx, li
 			block.p(changed, child_ctx);
 		}
 
-		new_blocks[i] = new_lookup[key] = block;
+		new_lookup.set(key, new_blocks[i] = block);
 
-		if (key in old_indexes) deltas[key] = Math.abs(i - old_indexes[key]);
+		if (key in old_indexes) deltas.set(key, Math.abs(i - old_indexes[key]));
 	}
 
-	const will_move = {};
+	const will_move = new Set();
-	const did_move = {};
+	const did_move = new Set();
 
 	function insert(block) {
 		if (block.i) block.i(1);
 		block.m(node, next);
-		lookup[block.key] = block;
+		lookup.set(block.key, block);
 		next = block.first;
 		n--;
 	}
@@ -72,32 +72,32 @@ export function update_keyed_each(old_blocks, changed, get_key, dynamic, ctx, li
 			n--;
 		}
 
-		else if (!new_lookup[old_key]) {
+		else if (!new_lookup.has(old_key)) {
 			// remove old block
 			destroy(old_block, lookup);
 			o--;
 		}
 
-		else if (!lookup[new_key] || will_move[new_key]) {
+		else if (!lookup.has(new_key) || will_move.has(new_key)) {
 			insert(new_block);
 		}
 
-		else if (did_move[old_key]) {
+		else if (did_move.has(old_key)) {
 			o--;
 
-		} else if (deltas[new_key] > deltas[old_key]) {
+		} else if (deltas.get(new_key) > deltas.get(old_key)) {
-			did_move[new_key] = true;
+			did_move.add(new_key);
 			insert(new_block);
 
 		} else {
-			will_move[old_key] = true;
+			will_move.add(old_key);
 			o--;
 		}
 	}
 
 	while (o--) {
 		const old_block = old_blocks[o];
-		if (!new_lookup[old_block.key]) destroy(old_block, lookup);
+		if (!new_lookup.has(old_block.key)) destroy(old_block, lookup);
 	}
 
 	while (n) insert(new_blocks[n - 1]);

test/js/samples/each-block-keyed-animated/expected.js

@@ -2,7 +2,6 @@
 import {
 	SvelteComponent,
 	append,
-	blank_object,
 	create_animation,
 	detach,
 	element,
@@ -73,7 +72,7 @@ function create_each_block(key_1, ctx) {
 }
 
 function create_fragment(ctx) {
-	var each_blocks = [], each_1_lookup = blank_object(), each_1_anchor;
+	var each_blocks = [], each_1_lookup = new Map(), each_1_anchor;
 
 	var each_value = ctx.things;
 
@@ -82,7 +81,7 @@ function create_fragment(ctx) {
 	for (var i = 0; i < each_value.length; i += 1) {
 		let child_ctx = get_each_context(ctx, each_value, i);
 		let key = get_key(child_ctx);
-		each_blocks[i] = each_1_lookup[key] = create_each_block(key, child_ctx);
+		each_1_lookup.set(key, each_blocks[i] = create_each_block(key, child_ctx));
 	}
 
 	return {

test/js/samples/each-block-keyed/expected.js

@@ -2,7 +2,6 @@
 import {
 	SvelteComponent,
 	append,
-	blank_object,
 	destroy_block,
 	detach,
 	element,
@@ -57,7 +56,7 @@ function create_each_block(key_1, ctx) {
 }
 
 function create_fragment(ctx) {
-	var each_blocks = [], each_1_lookup = blank_object(), each_1_anchor;
+	var each_blocks = [], each_1_lookup = new Map(), each_1_anchor;
 
 	var each_value = ctx.things;
 
@@ -66,7 +65,7 @@ function create_fragment(ctx) {
 	for (var i = 0; i < each_value.length; i += 1) {
 		let child_ctx = get_each_context(ctx, each_value, i);
 		let key = get_key(child_ctx);
-		each_blocks[i] = each_1_lookup[key] = create_each_block(key, child_ctx);
+		each_1_lookup.set(key, each_blocks[i] = create_each_block(key, child_ctx));
 	}
 
 	return {

test/runtime/samples/each-block-keyed-object-identity/_config.js

@@ -0,0 +1,25 @@
+export default {
+	props: {
+		todos: [
+			{ description: 'implement keyed each blocks' },
+			{ description: 'implement client-side hydration' }
+		]
+	},
+
+	html: `
+		<p>1: implement keyed each blocks</p>
+		<p>2: implement client-side hydration</p>
+	`,
+
+	test({ assert, component, target }) {
+		const [p1, p2] = target.querySelectorAll('p');
+
+		component.todos = [component.todos[1]];
+		assert.htmlEqual(target.innerHTML, '<p>1: implement client-side hydration</p>');
+
+		const [p3] = target.querySelectorAll('p');
+
+		assert.ok(!target.contains(p1), 'first <p> element should be removed');
+		assert.equal(p2, p3, 'second <p> element should be retained');
+	}
+};

test/runtime/samples/each-block-keyed-object-identity/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	export let todos;
+</script>
+
+{#each todos as todo, i (todo)}
+	<p>{i+1}: {todo.description}</p>
+{/each}