impr: output empty value placeholder

Author: nalgeon

src/output.js

@@ -13,6 +13,9 @@ const OutputMode = {
     hidden: "hidden",
 };
 
+// placeholder for the empty output value
+const PLACEHOLDER = "ok";
+
 const template = document.createElement("template");
 template.innerHTML = `
 <a href="#close">✕</a>
@@ -24,7 +27,7 @@ template.innerHTML = `
 const builders = {
     // returns the result as a text node.
     [OutputMode.text]: (result) => {
-        const text = result.stdout || result.stderr;
+        const text = result.stdout || result.stderr || PLACEHOLDER;
         return document.createTextNode(text);
     },
 

tests/snippet.js

@@ -55,6 +55,7 @@ async function runTests() {
     await testOutputModeIframe();
     await testOutputModeDOM();
     await testOutputModeHidden();
+    await testOutputPlaceholder();
 
     await testTemplate();
     await testTemplateChange();
@@ -743,6 +744,22 @@ async function testOutputModeHidden() {
     });
 }
 
+async function testOutputPlaceholder() {
+    return new Promise((resolve, reject) => {
+        t.log("testOutputPlaceholder...");
+        const ui = createSnippet(`
+            <pre><code>const a = 42;</code></pre>
+            <codapi-snippet engine="browser" sandbox="javascript" output-mode="text">
+            </codapi-snippet>
+        `);
+        ui.snip.addEventListener("result", (event) => {
+            t.assert("output", ui.output.out.innerHTML == "ok");
+            resolve();
+        });
+        ui.toolbar.run.click();
+    });
+}
+
 async function testTemplate() {
     return new Promise((resolve, reject) => {
         t.log("testTemplate...");