feat: allow missing default attribute for let tag

.changeset/metal-boxes-wash.md

@@ -0,0 +1,5 @@
+---
+"@marko/tags-api-preview": patch
+---
+
+Allow missing default attribute for let tag.

src/__tests__/fixtures/let/__snapshots__/let-no-default-attr/node.render.expected.html

@@ -0,0 +1,3 @@
+<div>
+  undefined
+</div>

src/__tests__/fixtures/let/__snapshots__/let-no-default-attr/web.render.expected.html

@@ -0,0 +1,3 @@
+<div>
+  undefined
+</div>

src/__tests__/fixtures/let/index.test.ts

@@ -125,6 +125,8 @@ describe(
   ])
 );
 
+describe("<let> no default attr", fixture("./templates/no-default-attr.marko"));
+
 describe("<let> error args", fixture("./templates/error-args.marko"));
 
 describe(
@@ -147,11 +149,6 @@ describe(
   fixture("./templates/error-extra-attr.marko")
 );
 
-describe(
-  "<let> error no default attr",
-  fixture("./templates/error-no-default-attr.marko")
-);
-
 describe(
   "<let> error no tag var",
   fixture("./templates/error-no-tag-var.marko")

src/__tests__/fixtures/let/templates/no-default-attr.marko

@@ -0,0 +1,2 @@
+<let/value/>
+<div>${typeof value}</div>

src/components/let/translate.ts

@@ -7,7 +7,7 @@ export = function translate(tag: t.NodePath<t.MarkoTag>) {
   const { file } = tag.hub;
   const server = file.markoOpts.output === "html";
   const tagVar = tag.node.var as t.Identifier;
-  let defaultAttr!: t.NodePath<t.MarkoAttribute>;
+  let defaultAttr: t.NodePath<t.MarkoAttribute> | undefined;
   let changeAttr: t.NodePath<t.MarkoAttribute> | undefined;
   let errorMessage: string | undefined;
 
@@ -33,8 +33,6 @@ export = function translate(tag: t.NodePath<t.MarkoTag>) {
       ? "requires a tag variable"
       : !t.isIdentifier(tagVar)
       ? "tag variable cannot be destructured"
-      : !defaultAttr
-      ? "must be initialized with a value"
       : tag.node.body.body.length
       ? "does not support body content"
       : tag.node.body.params.length
@@ -48,13 +46,16 @@ export = function translate(tag: t.NodePath<t.MarkoTag>) {
   }
 
   file.path.scope.crawl();
+  const defaultValue = defaultAttr
+    ? defaultAttr.node.value
+    : t.unaryExpression("void", t.numericLiteral(0));
   const binding = tag.scope.getBinding(tagVar.name)!;
 
   if (server) {
     file.path.scope.crawl();
     tag.replaceWith(
       t.variableDeclaration("const", [
-        t.variableDeclarator(tagVar, deepFreeze(file, defaultAttr.node.value)),
+        t.variableDeclarator(tagVar, deepFreeze(file, defaultValue)),
       ])
     );
   } else {
@@ -67,7 +68,7 @@ export = function translate(tag: t.NodePath<t.MarkoTag>) {
       t.assignmentExpression(
         "=",
         t.memberExpression(meta.state, keyString, true),
-        deepFreeze(file, defaultAttr.node.value)
+        deepFreeze(file, defaultValue)
       )
     );
 
@@ -82,7 +83,7 @@ export = function translate(tag: t.NodePath<t.MarkoTag>) {
 
       if (t.isFunction(changeAttr.node.value)) {
         setFnId = changeFnId;
-        decls.push(t.variableDeclarator(tagVar, defaultAttr.node.value));
+        decls.push(t.variableDeclarator(tagVar, defaultValue));
       } else {
         setFnId = tag.scope.generateUidIdentifier(`${tagVar.name}Set`);
         decls.push(
@@ -102,11 +103,7 @@ export = function translate(tag: t.NodePath<t.MarkoTag>) {
           ),
           t.variableDeclarator(
             tagVar,
-            t.conditionalExpression(
-              changeFnId,
-              defaultAttr.node.value,
-              getStateExpr
-            )
+            t.conditionalExpression(changeFnId, defaultValue, getStateExpr)
           )
         );
       }

src/util/deep-freeze/transform.ts

@@ -2,7 +2,12 @@ import { types as t } from "@marko/compiler";
 import { importDefault } from "@marko/babel-utils";
 
 export default (file: t.BabelFile, value: t.Expression) => {
-  if (file.markoOpts.optimize || t.isLiteral(value)) {
+  if (
+    file.markoOpts.optimize ||
+    t.isLiteral(value) ||
+    t.isBinaryExpression(value) ||
+    t.isUnaryExpression(value)
+  ) {
     return value;
   }