templates: Add {{#let}} block helper.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

frontend_tests/node_tests/templates.js

@@ -28,6 +28,21 @@ run_test("or", () => {
     assert.equal(html, "\n<p>last or</p>\n<p>true or</p>\n");
 });
 
+run_test("let", () => {
+    const html = require("./templates/let.hbs")({
+        outer_var: "hello",
+    });
+    assert.equal(
+        html,
+        `\
+    outer_var = hello
+    keyword_var = &lt;b&gt;escaped&lt;/b&gt;
+    block_var =     <b>unescaped</b> with hello
+
+`,
+    );
+});
+
 run_test("rendered_markdown", () => {
     const html = require("./templates/rendered_markdown.hbs")();
     const expected_html =

frontend_tests/node_tests/templates/let.hbs

@@ -0,0 +1,8 @@
+{{#let keyword_var="<b>escaped</b>"}}
+    {{#*inline "block_var"}}
+    <b>unescaped</b> with {{outer_var}}
+    {{/inline}}
+    outer_var = {{outer_var}}
+    keyword_var = {{keyword_var}}
+    block_var = {{block_var}}
+{{/let}}

static/js/templates.js

@@ -48,6 +48,26 @@ Handlebars.registerHelper({
     not(a) {
         return !a || Handlebars.Utils.isEmpty(a);
     },
+    ["let"](options) {
+        // Defines locally scoped variables.
+        // Example usage:
+        //     {{#let name1="value1"}}
+        //         {{#*inline "name2"}}
+        //         value2
+        //         {{/inline}}
+        //         Now {{name1}} and {{name2}} are in scope.
+        //     {{/let}}
+        return options.fn({
+            ...this,
+            ...options.hash,
+            ...Object.fromEntries(
+                Object.entries(options.fn.partials ?? {}).map(([name, value]) => [
+                    name,
+                    new Handlebars.SafeString(value(this)),
+                ]),
+            ),
+        });
+    },
 });
 
 // Note that this i18n caching strategy does not allow us to support

tools/lib/template_parser.py

@@ -133,6 +133,8 @@ def tokenize(text: str) -> List[Token]:
             elif looking_at_handlebars_start():
                 s = get_handlebars_tag(text, state.i)
                 tag = s[3:-2].split()[0]
+                if tag.startswith("*"):
+                    tag = tag[1:]
                 kind = "handlebars_start"
             elif looking_at_handlebars_end():
                 s = get_handlebars_tag(text, state.i)

webpack.config.ts

@@ -148,6 +148,7 @@ export default (_env: unknown, argv: {mode?: string}): webpack.Configuration[] =
                                     "and",
                                     "or",
                                     "not",
+                                    "let",
                                     "t",
                                     "tr",
                                     "rendered_markdown",