Add empty filter

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;
+    };
+  };
+}

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());
   };
 }

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",
+  });
+});