Fixes #188 #3345

src/Template.js

@@ -637,8 +637,77 @@ class Template extends TemplateContent {
 		});
 	}
 
+	static async runPreprocessors(inputPath, content, data, preprocessors) {
+		let skippedVia = false;
+		for (let [name, preprocessor] of Object.entries(preprocessors)) {
+			let { filter, callback } = preprocessor;
+
+			let filters;
+			if (Array.isArray(filter)) {
+				filters = filter;
+			} else if (typeof filter === "string") {
+				filters = filter.split(",");
+			} else {
+				throw new Error(
+					`Expected file extensions passed to "${name}" content preprocessor to be a string or array. Received: ${filter}`,
+				);
+			}
+
+			if (!filters.some((extension) => extension === "*" || inputPath.endsWith(extension))) {
+				// skip
+				continue;
+			}
+
+			try {
+				let ret = await callback.call(
+					{
+						inputPath,
+					},
+					data,
+					content,
+				);
+
+				// Returning explicit false is the same as ignoring the template
+				if (ret === false) {
+					skippedVia = name;
+					continue;
+				}
+
+				// Different from transforms: returning falsy (not false) here does nothing (skips the preprocessor)
+				if (ret) {
+					content = ret;
+				}
+			} catch (e) {
+				throw new EleventyBaseError(
+					`Preprocessor \`${name}\` encountered an error when transforming ${inputPath}.`,
+					e,
+				);
+			}
+		}
+
+		return {
+			skippedVia,
+			content,
+		};
+	}
+
 	async getTemplates(data) {
-		let rawInput = await this.getPreRender();
+		let content = await this.getPreRender();
+		let { skippedVia, content: rawInput } = await Template.runPreprocessors(
+			this.inputPath,
+			content,
+			data,
+			this.config.preprocessors,
+		);
+
+		if (skippedVia) {
+			debug(
+				"Skipping %o, the %o preprocessor returned an explicit `false`",
+				this.inputPath,
+				skippedVia,
+			);
+			return [];
+		}
 
 		// https://github.com/11ty/eleventy/issues/1206
 		data.page.rawInput = rawInput;

src/TemplateConfig.js

@@ -333,6 +333,8 @@ class TemplateConfig {
 		}
 
 		this.userConfig.activeNamespace = storedActiveNamespace;
+
+		this.userConfig._disablePluginExecution();
 	}
 
 	/**

src/UserConfig.js

@@ -102,6 +102,8 @@ class UserConfig {
 
 		this.linters = {};
 		this.transforms = {};
+		this.preprocessors = {};
+
 		this.activeNamespace = "";
 		this.DateTime = DateTime;
 		this.dynamicPermalinks = true;
@@ -544,6 +546,11 @@ class UserConfig {
 		this.#pluginExecution = true;
 	}
 
+	// Internal method
+	_disablePluginExecution() {
+		this.#pluginExecution = false;
+	}
+
 	/* Config is executed in two stages and plugins are the second stage—are we in the plugins stage? */
 	isPluginExecution() {
 		return this.#pluginExecution;
@@ -999,6 +1006,15 @@ class UserConfig {
 		this.transforms[name] = this.#decorateCallback(`"${name}" Transform`, callback);
 	}
 
+	addPreprocessor(name, fileExtensions, callback) {
+		name = this.getNamespacedName(name);
+
+		this.preprocessors[name] = {
+			filter: fileExtensions,
+			callback: this.#decorateCallback(`"${name}" Preprocessor`, callback),
+		};
+	}
+
 	addLinter(name, callback) {
 		name = this.getNamespacedName(name);
 
@@ -1055,6 +1071,7 @@ class UserConfig {
 			// filters removed in 1.0 (use addTransform instead)
 			transforms: this.transforms,
 			linters: this.linters,
+			preprocessors: this.preprocessors,
 			globalData: this.globalData,
 			layoutAliases: this.layoutAliases,
 			layoutResolution: this.layoutResolution,

test/EleventyTest.js

@@ -1527,3 +1527,23 @@ test("#1419: Shortcode in a permalink", async (t) => {
   t.is(results.length, 1);
   t.is(results[0].url, `/url-slug/`);
 });
+
+test("#188: Content preprocessing", async (t) => {
+  let elev = new Eleventy("./test/stubs-virtual/", undefined, {
+    config: eleventyConfig => {
+      eleventyConfig.addPreprocessor("drafts", ".njk", (data, content) => {
+        if(data.draft) {
+          return false;
+        }
+        return `Hello ${content}`;
+      });
+
+      eleventyConfig.addTemplate("index.njk", "Before");
+      eleventyConfig.addTemplate("draft.njk", "Before", { draft: true });
+    }
+  });
+
+  let results = await elev.toJSON();
+  t.is(results.length, 1);
+  t.is(results[0].content, `Hello Before`);
+});