Nunjucks task with gulp

gistfile1.txt

gulp.task('nunjucks', function() {
    return gulp.src('./source/html/pages/*.html')
    .pipe(data(function() {
      return JSON.parse(fs.readFileSync('./source/data/data.json'));
    }))
        .pipe(nunjucks(nunjucksOpts))
        .pipe(gulp.dest('build'))
        .pipe(browserSync.stream());
});