fix dev mode each block validation when using strings (#4451)

Author: Conduitry

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Svelte changelog
 
+## Unreleased
+
+* Fix dev mode validation of `{#each}` blocks using strings ([#4450](https://github.com/sveltejs/svelte/issues/4450))
+
 ## 3.19.0
 
 * Fix indirect bindings involving elements with spreads ([#3680](https://github.com/sveltejs/svelte/issues/3680))

src/runtime/internal/dev.ts

@@ -80,7 +80,7 @@ export function set_data_dev(text, data) {
 }
 
 export function validate_each_argument(arg) {
-	if (!arg || !('length' in arg)) {
+	if (typeof arg !== 'string' && !(arg && typeof arg === 'object' && 'length' in arg)) {
 		let msg = '{#each} only iterates over array-like objects.';
 		if (typeof Symbol === 'function' && arg && Symbol.iterator in arg) {
 			msg += ' You can use a spread to convert this iterable into an array.';

test/runtime/samples/each-block-string/_config.js

@@ -0,0 +1,10 @@
+export default {
+	compileOptions: {
+		dev: true
+	},
+	html: `
+		<div>f</div>
+		<div>o</div>
+		<div>o</div>
+	`
+};

test/runtime/samples/each-block-string/main.svelte

@@ -0,0 +1,3 @@
+{#each 'foo' as c}
+	<div>{c}</div>
+{/each}