Technically it's a "yesterday I learned" kind of post

_posts/2021-07-05-accessing-variables-in-handlebars-rust.md

@@ -0,0 +1,61 @@
+---
+layout: post
+title: "Accessing Handlebars variables in an outer scope"
+tags:
+- rust
+---
+
+This weekend I learned some unfamiliar behaviors with the way Handlebars
+handles nested variable scopes. I typically use Handlebars via the
+[handlebars-rust](https://github.com/sunng87/handlebars-rust) implementation
+which aims to maintain nearly one to one compatibility with the [JavaScript
+implementation](https://handlebarsjs.com/). They have block scope helpers such
+as `#each` and `#with`, both of which create an inner scope for variable
+resolution. Unfortunately, the syntax can be quite unintuitive for accessing outer
+scope once in those nested scopes.
+
+Handlebars is a largely declarative templating syntax which uses curlybraces
+such as `{{var}}` for variable and helper substitution. The `#each` helper is
+important for loops, imagine the following data structure:
+
+```json
+{
+  "repos" : [
+    {
+      "name" : "otto"
+    },
+    {
+      "name" : "l4bsd"
+    }
+  ],
+  "mood" : "cool"
+}
+```
+
+This could be rendered into a list on a page via:
+
+```html
+<ul>{% raw %}
+    {{#each data.repos}}
+        <li>\{{name}}</li>
+    {{/each}}{% endraw %}
+</ul>
+```
+
+Inside the `#each` block the values of the indexed object become the scope for variable resolution, such that `{{name}}` actually refers to `data.repos[i].name`. This presents problems when the template must refer to outer scope variables, such as `mood`. In the Rust implementation this variable resolution can be accomplished through a path traversal style syntax such as:
+
+```html
+<ul>{% raw %}
+    {{#each data.repos}}
+        <li>\{{name}} is {{../data.mood}}</li>
+    {{/each}}{% endraw %}
+</ul>
+```
+
+The `../data.mood` is all that's needed to refer to the variable in the outer
+scope of variables. Not what I expected at all, and the only reason I found it
+was because I found [an old
+issue](https://github.com/sunng87/handlebars-rust/issues/416) which alluded to
+the syntax and gave it a try.
+
+