Add {{ default }} tag

plugins/auto_trim.ts

@@ -5,6 +5,8 @@ export const defaultTags = [
   ">",
   "set",
   "/set",
+  "default",
+  "/default",
   "if",
   "/if",
   "else",

plugins/default.ts

@@ -0,0 +1,56 @@
+import { SourceError } from "../core/errors.ts";
+import type { Token } from "../core/tokenizer.ts";
+import type { Environment, Plugin } from "../core/environment.ts";
+
+export default function (): Plugin {
+  return (env: Environment) => {
+    env.tags.push(defaultTag);
+  };
+}
+
+const VARNAME = /^[a-zA-Z_$][\w$]*$/;
+const VALID_TAG = /^([a-zA-Z_$][\w$]*)\s*=\s*([^]+)$/;
+
+function defaultTag(
+  env: Environment,
+  token: Token,
+  _output: string,
+  tokens: Token[],
+): string | undefined {
+  const [, code, position] = token;
+  const { dataVarname } = env.options;
+
+  if (!code.startsWith("default ")) {
+    return;
+  }
+
+  const expression = code.replace("default", "").trim();
+  // Setting a value (e.g. {{ default foo = "bar" }}
+  if (expression.includes("=")) {
+    const match = expression.match(VALID_TAG);
+    if (!match) {
+      throw new SourceError("Invalid default tag", position);
+    }
+    const variable = match[1];
+    const value = env.compileFilters(tokens, match[2]);
+    return `
+      if (typeof ${variable} == "undefined" || ${variable} === null) {
+        var ${variable} = ${dataVarname}["${variable}"] = ${value};
+      }
+    `;
+  }
+
+  // Capture a value (e.g. {{ default foo }}bar{{ /default }}
+  if (!VARNAME.test(expression)) {
+    throw new SourceError("Invalid default tag", position);
+  }
+  const subvarName = `${dataVarname}["${expression}"]`;
+  const compiledFilters = env.compileFilters(tokens, subvarName);
+  return `
+    if (typeof ${expression} == "undefined" || ${expression} === null) {
+      ${subvarName} = "";
+      ${env.compileTokens(tokens, subvarName, "/default").join("")}
+      var ${expression} = ${subvarName} = ${compiledFilters};
+    }
+  `;
+}

plugins/mod.ts

@@ -4,6 +4,7 @@ import ifTag from "./if.ts";
 import forTag from "./for.ts";
 import includeTag from "./include.ts";
 import setTag from "./set.ts";
+import defaultTag from "./default.ts";
 import jsTag from "./js.ts";
 import layoutTag from "./layout.ts";
 import functionTag from "./function.ts";
@@ -21,6 +22,7 @@ export default function (): Plugin {
     env.use(jsTag());
     env.use(includeTag());
     env.use(setTag());
+    env.use(defaultTag());
     env.use(layoutTag());
     env.use(functionTag());
     env.use(importTag());

test/default.test.ts

@@ -0,0 +1,61 @@
+import { test } from "./utils.ts";
+
+Deno.test("Default tag", async () => {
+  await test({
+    template: `
+    {{ set greeting = "Hello" }}
+    {{ default message = "Hi" }}
+    {{ default target = "world" }}
+    {{ greeting }} {{ target }}
+    `,
+    expected: "Hello world",
+  });
+
+  await test({
+    template: `
+    {{ default message -}}
+      Hi
+    {{- /default }}
+    {{ default target -}}
+      world
+    {{- /default }}
+    {{ greeting }} {{ target }}
+    `,
+    expected: "Hello world",
+    data: {
+      greeting: "Hello",
+      target: null,
+    },
+  });
+});
+
+Deno.test("Default tag in strict mode", async () => {
+  await test({
+    options: { strict: true },
+    template: `
+    {{ set greeting = "Hello" }}
+    {{ default message = "Hi" }}
+    {{ default target = "world" }}
+    {{ greeting }} {{ target }}
+    `,
+    expected: "Hello world",
+  });
+
+  await test({
+    options: { strict: true },
+    template: `
+    {{ default message -}}
+      Hi
+    {{- /default }}
+    {{ default target -}}
+      world
+    {{- /default }}
+    {{ greeting }} {{ target }}
+    `,
+    expected: "Hello world",
+    data: {
+      greeting: "Hello",
+      target: null,
+    },
+  });
+});