Add support for async filters (#37)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

index.js

@@ -2,6 +2,7 @@
 const through = require('through2');
 const nunjucks = require('nunjucks');
 const PluginError = require('plugin-error');
+const is = require('@sindresorhus/is');
 
 function compile(data, options = {}) {
 	return through.obj(function (file, encoding, callback) {
@@ -19,21 +20,54 @@ function compile(data, options = {}) {
 		const filePath = file.path;
 		const env = options.env || new nunjucks.Environment(new nunjucks.FileSystemLoader(file.base), options);
 
+		let isAsync = false;
+
 		if (options.filters && !options.env) {
 			for (const key of Object.keys(options.filters)) {
-				env.addFilter(key, options.filters[key]);
+				const filter = options.filters[key];
+				if (is.asyncFunction(filter)) {
+					isAsync = true;
+					env.addFilter(key, async (...args) => {
+						const cb = args.pop();
+						try {
+							const result = await filter(...args);
+							cb(null, result);
+						} catch (error) {
+							cb(error, null);
+						}
+					}, true);
+				} else {
+					env.addFilter(key, filter);
+				}
 			}
 		}
 
 		try {
-			file.contents = Buffer.from(env.renderString(file.contents.toString(), context));
-			file.extname = '.html';
-			this.push(file);
+			const writeResult = result => {
+				file.contents = Buffer.from(result);
+				file.extname = '.html';
+				this.push(file);
+			};
+
+			if (isAsync) {
+				env.renderString(file.contents.toString(), context, (error, result) => {
+					if (error) {
+						this.emit('error', new PluginError('gulp-nunjucks', error, {fileName: filePath}));
+						callback();
+						return;
+					}
+
+					writeResult(result);
+					callback();
+				});
+			} else {
+				writeResult(env.renderString(file.contents.toString(), context));
+				callback();
+			}
 		} catch (error) {
 			this.emit('error', new PluginError('gulp-nunjucks', error, {fileName: filePath}));
+			callback();
 		}
-
-		callback();
 	});
 }
 

package.json

@@ -33,6 +33,7 @@
 		"javascript"
 	],
 	"dependencies": {
+		"@sindresorhus/is": "^3.1.2",
 		"nunjucks": "^3.2.0",
 		"plugin-error": "^1.0.1",
 		"through2": "^3.0.1"

readme.md

@@ -87,12 +87,19 @@ Type: `object`
 
 An object containing [custom filters](https://mozilla.github.io/nunjucks/api.html#custom-filters) that will be passed to Nunjucks, with the filter's name as key and the filter function as value.
 
+Async filters should be defined as async functions. You cannot use just a promise-returning function.
+
 Example:
 
 ```js
 {
 	'shorten': string => string.slice(0, 5),
-	'round': number => Math.round(number)
+	'round': number => Math.round(number),
+	'fetch': async url => {
+		const response = await fetch(url);
+		const result = await response.text();
+		return result;
+	}
 }
 ```
 

test.js

@@ -194,6 +194,22 @@ test.cb('support custom filters', t => {
 	}));
 });
 
+test.cb('support async custom filters', t => {
+	const filters = {shorten: async x => x.slice(0, 5), shout: async x => `${x}!`};
+
+	const stream = nunjucks.compile({message: 'Lorem ipsum'}, {filters});
+
+	stream.on('data', file => {
+		t.is(file.contents.toString(), 'Lorem!');
+		t.end();
+	});
+
+	stream.end(new Vinyl({
+		path: 'foo.txt',
+		contents: Buffer.from('{{ message|shorten|shout }}')
+	}));
+});
+
 test.cb('not pass custom filters to custom environment', t => {
 	const nunjucksModule = require('nunjucks');