From 835b22c4b736219f1fac6c62fee7724fb228f587 Mon Sep 17 00:00:00 2001
From: vrugtehagel <vrugtehagel@pm.me>
Date: Fri, 19 Dec 2025 10:15:55 +0100
Subject: [PATCH] Add `empty` filter

I considered `isEmpty` (and `is_empty`) as well, but I went with the
simpler `empty` because its use is mostly limited to `{{ if }}`
statements. It's unlikely users are directly printing booleans, and the
filter is more-or-less useless in tags like `{{ echo }}` or `{{ for }}`.
Given the relatively narrow scope, I don't think there are readability
issues with omitting the `is` from the name. Generally it'll be used in
contexts where booleans are expected, like

```vto
{{ if items |> empty }}
  {{ set isMissingTags = tags |> empty }}
{{ /if }}
```
---
 plugins/empty.ts   | 16 ++++++++++
 plugins/mod.ts     |  2 ++
 test/empty.test.ts | 73 ++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 91 insertions(+)
 create mode 100644 plugins/empty.ts
 create mode 100644 test/empty.test.ts

diff --git a/plugins/empty.ts b/plugins/empty.ts
new file mode 100644
index 0000000..e557cd1
--- /dev/null
+++ b/plugins/empty.ts
@@ -0,0 +1,16 @@
+import { SafeString } from "../core/environment.ts";
+import type { Environment, Plugin } from "../core/environment.ts";
+
+export default function (): Plugin {
+  return (env: Environment) => {
+    env.filters.empty = (value: unknown): boolean => {
+      if (!value) return true;
+      if (typeof value == "string" || value instanceof SafeString) {
+        return value.toString().trim() == "";
+      }
+      if (typeof value != "object") return false;
+      if (Array.isArray(value)) return value.length == 0;
+      return false;
+    };
+  };
+}
diff --git a/plugins/mod.ts b/plugins/mod.ts
index 0701937..b1a378f 100644
--- a/plugins/mod.ts
+++ b/plugins/mod.ts
@@ -14,6 +14,7 @@ import echoTag from "./echo.ts";
 import escape from "./escape.ts";
 import unescape from "./unescape.ts";
 import trim from "./trim.ts";
+import empty from "./empty.ts";
 
 export default function (): Plugin {
   return (env: Environment) => {
@@ -31,5 +32,6 @@ export default function (): Plugin {
     env.use(escape());
     env.use(unescape());
     env.use(trim());
+    env.use(empty());
   };
 }
diff --git a/test/empty.test.ts b/test/empty.test.ts
new file mode 100644
index 0000000..05c94cb
--- /dev/null
+++ b/test/empty.test.ts
@@ -0,0 +1,73 @@
+import { test } from "./utils.ts";
+
+Deno.test("Escape filter", async () => {
+  await test({
+    template: `
+    NaN: {{ NaN |> empty }}
+    `,
+    expected: "NaN: true",
+  });
+
+  await test({
+    template: `
+    Zero: {{ 0 |> empty }}
+    `,
+    expected: "Zero: true",
+  });
+
+  await test({
+    template: `
+    One: {{ 1 |> empty }}
+    `,
+    expected: "One: false",
+  });
+
+  await test({
+    template: `
+    Null: {{ null |> empty }}
+    `,
+    expected: "Null: true",
+  });
+
+  await test({
+    template: `
+    Empty object: {{ {} |> empty }}
+    `,
+    expected: "Empty object: false",
+  });
+
+  await test({
+    template: `
+    Empty array: {{ [] |> empty }}
+    `,
+    expected: "Empty array: true",
+  });
+
+  await test({
+    template: `
+    Array with empty slot: {{ Array(1) |> empty }}
+    `,
+    expected: "Array with empty slot: false",
+  });
+
+  await test({
+    template: `
+    Empty string: {{ "" |> empty }}
+    `,
+    expected: "Empty string: true",
+  });
+
+  await test({
+    template: String.raw`
+    Whitespace only: {{ " \n\t " |> empty }}
+    `,
+    expected: "Whitespace only: true",
+  });
+
+  await test({
+    template: `
+    String of zero: {{ "0" |> empty }}
+    `,
+    expected: "String of zero: false",
+  });
+});