fix: improve error messages in case of invalid variable names (#32)

* fix: improve error messages in case of invalid variable names

Fixes #29.

* fix: update error message and readme

Author: nishantwrp

README.md

@@ -94,6 +94,8 @@ The currently supported custom variable types are:
 | `boolean` | show_summary: boolean |
 | `enum` (dropdown list) | color: enum(Red, Yellow) |
 
+You can't use special characters ("@", ",", "#", "+", "(", etc.) or spaces in variable names. However, you can use "_" in variable names.
+
 > **NOTE**: If you declare a custom variable with same name as the built-in variables, the custom variable value will be used.
 
 Internally, [Handlebars.Js](https://handlebarsjs.com/) is used to compile the templates. You can write templates to be compatible with `Handlebars`.

src/parser.ts

@@ -59,11 +59,24 @@ export class Parser {
         return variableValues;
     }
 
+    private checkVariableNames(variableNames: string[]) {
+        for (const variable of variableNames) {
+            try {
+                const compiledBody = Handlebars.compile(`{{ ${variable} }}`);
+                compiledBody({});
+            } catch {
+                throw new Error(`Variable name "${variable}" is invalid.\n\nPlease avoid using special characters ("@", ",", "#", "+", "(", etc.) or spaces in variable names. However, you can use "_" in variable names.`);
+            }
+        }
+    }
+
     private async getVariableInputs(title: string, variables: Record<string, string>) {
         if (Object.keys(variables).length == 0) {
             return {};
         }
 
+        this.checkVariableNames(Object.keys(variables));
+
         await setTemplateVariablesView(this.dialog, title, variables);
         const dialogResponse = (await joplin.views.dialogs.open(this.dialog));