Add frontmatter helpers

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@foundry/hammerstone",
-  "version": "0.3.0",
+  "version": "0.3.1",
   "description": "Load and manipulate Obsidian vault data",
   "type": "module",
   "exports": "./bin/hammerstone.js",

src/document.ts

@@ -177,6 +177,54 @@ export default class MarkdownDocument {
     return this;
   }
 
+  /**
+   * Retrieves the value of a frontmatter key as a string.
+   * If the key is not found, returns the provided default value.
+   * Throws an error if the value is not a string.
+   *
+   * @param key - The key of the frontmatter value to retrieve.
+   * @param defaultValue - The default value to return if the key is not found.
+   * @returns The value of the frontmatter key as a string.
+   * @throws Error if the frontmatter key is not a string.
+   */
+  getFrontmatterString(key: string, defaultValue: string): string {
+    const val = this._frontmatter[key];
+
+    if (val === undefined) {
+      return defaultValue;
+    }
+
+    if (typeof val !== "string") {
+      throw new Error(`Frontmatter key is not a string: ${key}`);
+    }
+
+    return val;
+  }
+
+  /**
+   * Retrieves an array value from the frontmatter object based on the provided key.
+   * If the value is not found, it returns the provided defaultValue.
+   * If the value is found but is not an array, it throws an error.
+   *
+   * @param key - The key to retrieve the array value from the frontmatter object.
+   * @param defaultValue - The default value to return if the key is not found in the frontmatter object.
+   * @returns The array value associated with the provided key, or the defaultValue if the key is not found.
+   * @throws Error if the value associated with the key is found but is not an array.
+   */
+  getFrontmatterArray(key: string, defaultValue: string[]): string[] {
+    const val = this._frontmatter[key];
+
+    if (val === undefined) {
+      return defaultValue;
+    }
+
+    if (!Array.isArray(val)) {
+      throw new Error(`Frontmatter key is not an array: ${key}`);
+    }
+
+    return val;
+  }
+
   /** The markdown portion of the file (without frontmatter) */
   get markdown() {
     return this._markdown;