Added some notes about observed behavior and fixed return types for missing/absent keys.

js/test_cases/helpers_size/README.md

@@ -7,3 +7,12 @@ Excerpt from [the DustJS tutorial](https://github.com/linkedin/dustjs/wiki/Dust-
     Undefined, 0, empty string - zero
     Any other value - length after conversion to string
 
+This appears to be inaccurate (probably out of date because that tutorial was deprecated in favor of [dustjs.com](https://www.dustjs.com/), but the latter is incomplete and lacking any reference to the `@size` helper.
+
+Corrections
+-----------
+- Booleans are 0, not converted to strings
+
+Oddities
+--------
+Reference parameters (like `foo="{bar}"`) are usually treated as strings but it seems if it contains ONLY a reference to a value and not anything else, then it is still treated as a number.

js/test_cases/helpers_size/inputobject.json

@@ -1,6 +1,6 @@
 {
   "val": {
-    "pet": "cat",
-    "name": "fluffy"
+    "name": "fluffy",
+    "pet": "cat"
   }
 }

js/test_cases/helpers_size/main.dust

@@ -1,3 +1,3 @@
 The size of val ({val|js|s}) is {@size key=val /}{~n}
-The size of "{val}" is {@size key="{val}" /}{~n}
+The size of "{~lb}val{~rb}" is {@size key="{val}" /}{~n}
 The size with no key is {@size /}{~n}

src/renderer/renderer.rs

@@ -733,7 +733,12 @@ impl<'a> DustRenderer<'a> {
                         .map(|ce| ce.get_size())
                 });
                 match value_ce {
-                    None | Some(Err(_)) | Some(Ok(None)) => return Ok("".to_owned()),
+                    // If we found the key but could not get the size from it, render nothing.
+                    Some(Ok(None)) => return Ok("".to_owned()),
+                    // If "key" is not on the @size tag at all, render 0.
+                    None => return Ok("0".to_owned()),
+                    // If the key value could not be found in the context, render 0.
+                    Some(Err(_)) => return Ok("0".to_owned()),
                     Some(Ok(Some(ce_size))) => {
                         return ce_size.get_context_element_reference().render(&Vec::new())
                     }