templates: Support context variables in Handlebars {{t}} helper.

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

docs/translating/internationalization.md

@@ -258,6 +258,8 @@ Handlebars [helpers][] that Zulip registers. The syntax for simple strings is:
 
 ```html+handlebars
 {{t 'English text' }}
+
+{{t 'Block of English text with a {variable}.' }}
 ```
 
 If you are passing a translated string to a Handlebars partial, you can use:
@@ -268,17 +270,17 @@ If you are passing a translated string to a Handlebars partial, you can use:
     }}
 ```
 
-The syntax for block strings or strings containing variables is:
+The syntax for HTML strings is:
 
 <!-- The html+handlebars lexer fails to lex the single braces. -->
 
 ```text
 {{#tr}}
-    Block of English text.
+    <p>Block of English text.</p>
 {{/tr}}
 
 {{#tr}}
-    Block of English text with a {variable}.
+    <p>Block of English text with a {variable}.</p>
 {{/tr}}
 ```
 

web/src/templates.js

@@ -51,13 +51,19 @@ Handlebars.registerHelper({
     },
 });
 
-Handlebars.registerHelper("t", (message) => {
+Handlebars.registerHelper("t", function (message) {
     // Marks a string for translation.
-    // Example usage:
+    // Example usage 1:
     //     {{t "some English text"}}
+    //
+    // Example usage 2:
+    //     {{t "This {variable} will get value from the current context"}}
+    //
+    // Note: use `{` and `}` instead of `{{` and `}}` to declare
+    // variables.
 
     const descriptor = {id: message, defaultMessage: message};
-    return intl.formatMessage(descriptor);
+    return intl.formatMessage(descriptor, this);
 });
 
 Handlebars.registerHelper("tr", function (options) {
@@ -90,7 +96,7 @@ Handlebars.registerHelper("tr", function (options) {
             ]),
         ),
         ...Object.fromEntries(
-            Object.entries(this ?? {}).map(([key, value]) => [
+            Object.entries(this).map(([key, value]) => [
                 key,
                 Handlebars.Utils.escapeExpression(value),
             ]),