Handlebars render to HTML task for Gulp.js, with a little bit of the Harp metadata context spec implemented. (using ES6 arrow functions and string templates)

gulp-handlebars.js

'use strict';

var $          = require('gulp-load-plugins')();
var _          = require('lodash');
var fs         = require('node-fs-extra');
var gulp       = require('gulp');
var handlebars = require('handlebars');
var path       = require('path');

gulp.task('handlebars', () => {
    return gulp.src('*.hbs')
        .pipe($.tap(file => {
            var data     = {};
            var ext      = path.extname(file.path);
            var filename = file.path.replace(file.base, '');
            var resource = path.basename(file.path, ext);

            // According to the Node API using file exists methods are
            // deprecated because they're anti-patterns so it's suggested to
            // just attempt to open every file and wrap it in a try/catch.
            try {
                /**
                 * Parse metadata according to the Harp spec
                 * @see http://harpjs.com/docs/development/metadata
                 */
                data = fs.readJsonSync(`${file.base}_data.json`);
            } catch (e) {}

            var context = data.hasOwnProperty(resource) ? data[resource] : {};

            try {
                // Page specific metadata will overrule global data.
                var metadataFile = $.util.replaceExtension(file.path, '.json');
                _.merge(context, fs.readJsonSync(metadataFile));
            } catch (e) {}

            var html = handlebars.compile(file._contents.toString()).call(this, context);

            file._contents = new Buffer(html);
            file.path      = $.util.replaceExtension(file.path, '.html');

        }))
        .pipe(gulp.dest('tmp/'));
});