unique-id: Adjust uniqueId() implementation to only generate valid …

…selectors

Regular UUIDs are allowed to start with numeric digits, but CSS selectors may not start with those. This change adjusts the implementation to only return UUIDs starting with a letter instead of a digit.

packages/@ember/-internals/glimmer/lib/helpers/unique-id.ts

@@ -37,12 +37,14 @@ export default internalHelper((): Reference<string> => {
 // This code should be reasonably fast, and provide a unique value every time
 // it's called, which is what we need here. It produces a string formatted as a
 // standard UUID, which avoids accidentally turning Ember-specific
-// implementation details into an intimate API.
+// implementation details into an intimate API. It also ensures that the UUID
+// always starts with a letter, to avoid creating invalid IDs with a numeric
+// digit at the start.
 function uniqueId() {
   // @ts-expect-error this one-liner abuses weird JavaScript semantics that
   // TypeScript (legitimately) doesn't like, but they're nonetheless valid and
   // specced.
-  return ([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, (a) =>
-    (a ^ ((Math.random() * 16) >> (a / 4))).toString(16)
+  return ([3e7] + -1e3 + -4e3 + -2e3 + -1e11).replace(/[0-3]/g, (a) =>
+    ((a * 4) ^ ((Math.random() * 16) >> (a & 2))).toString(16)
   );
 }

packages/@ember/-internals/glimmer/tests/integration/helpers/unique-id-test.js

@@ -102,6 +102,24 @@ if (EMBER_UNIQUE_ID_HELPER) {
         });
       }
 
+      ['@test it only generates valid selectors']() {
+        let iterations = 1000;
+        let reNumericStart = /^\d/;
+
+        let template = '<p>{{unique-id}}</p>'.repeat(iterations);
+        super.render(template);
+
+        for (let i = 0; i < iterations; i++) {
+          let textNode = this.nthChild(i).firstChild;
+          let text = textNode.data;
+
+          this.assert.false(
+            reNumericStart.test(text),
+            `{{unique-id}} should produce valid selectors` + text
+          );
+        }
+      }
+
       render(template, ...rest) {
         // If there are three parameters to `render`, the second parameter is the
         // template's arguments.