Merge pull request #2424 from sveltejs/gh-2243

add tutorial chapter for mutations

site/content/tutorial/02-reactivity/04-updating-arrays-and-objects/app-a/App.svelte

@@ -0,0 +1,15 @@
+<script>
+	let numbers = [1, 2, 3, 4];
+
+	function addNumber() {
+		numbers.push(numbers.length + 1);
+	}
+
+	$: sum = numbers.reduce((t, n) => t + n, 0);
+</script>
+
+<p>{numbers.join(' + ')} = {sum}</p>
+
+<button on:click={addNumber}>
+	Add a number
+</button>

site/content/tutorial/02-reactivity/04-updating-arrays-and-objects/app-b/App.svelte

@@ -0,0 +1,15 @@
+<script>
+	let numbers = [1, 2, 3, 4];
+
+	function addNumber() {
+		numbers = [...numbers, numbers.length + 1];
+	}
+
+	$: sum = numbers.reduce((t, n) => t + n, 0);
+</script>
+
+<p>{numbers.join(' + ')} = {sum}</p>
+
+<button on:click={addNumber}>
+	Add a number
+</button>

site/content/tutorial/02-reactivity/04-updating-arrays-and-objects/text.md

@@ -0,0 +1,26 @@
+---
+title: Updating arrays and objects
+---
+
+Because Svelte's reactivity is triggered by assignments, using array methods like `push` and `splice` won't automatically cause updates. For example, clicking the button doesn't do anything.
+
+One way to fix that is to add an assignment that would otherwise be redundant:
+
+```js
+function addNumber() {
+	numbers.push(numbers.length + 1);
+	numbers = numbers;
+}
+```
+
+But there's a more *idiomatic* solution:
+
+```js
+function addNumber() {
+	numbers = [...numbers, numbers.length + 1;]
+}
+```
+
+You can use similar patterns to replace `pop`, `shift`, `unshift` and `splice`.
+
+> Assignments to *properties* of arrays and objects — e.g. `obj.foo += 1` or `array[i] = x` — work the same way as assignments to the values themselves.