Add frontmatter number

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@foundry/hammerstone",
-  "version": "0.3.1",
+  "version": "0.3.2",
   "description": "Load and manipulate Obsidian vault data",
   "type": "module",
   "exports": "./bin/hammerstone.js",

src/document.ts

@@ -194,11 +194,7 @@ export default class MarkdownDocument {
       return defaultValue;
     }
 
-    if (typeof val !== "string") {
-      throw new Error(`Frontmatter key is not a string: ${key}`);
-    }
-
-    return val;
+    return `${val}`;
   }
 
   /**
@@ -225,6 +221,30 @@ export default class MarkdownDocument {
     return val;
   }
 
+  /**
+   * Retrieves a number value from the frontmatter using the specified key.
+   * If the key is not found in the frontmatter, the defaultValue is returned.
+   * If the value associated with the key is not a number, an error is thrown.
+   *
+   * @param key - The key to retrieve the number value from the frontmatter.
+   * @param defaultValue - The default value to return if the key is not found in the frontmatter.
+   * @returns The number value associated with the key in the frontmatter, or the defaultValue if the key is not found.
+   * @throws Error if the value associated with the key is not a number.
+   */
+  getFrontmatterNumber(key: string, defaultValue: number): number {
+    const val = this._frontmatter[key];
+
+    if (val === undefined) {
+      return defaultValue;
+    }
+
+    if (typeof val !== "number") {
+      throw new Error(`Frontmatter key is not a number: ${key}`);
+    }
+
+    return val;
+  }
+
   /** The markdown portion of the file (without frontmatter) */
   get markdown() {
     return this._markdown;