Filter posts by a passed year

example.md

title	year	layout
An example	2018	example.njk

example.njk

<ul>
    {% for item in collections.writing | yearFilter(year) %}
        <li>
          <!-- item -->
        </li>
    {% endfor %}
</ul>

year-filter.js

/**
 * Filter and return content for the passed year
 *
 * @param {Array} value
 * @param {String|Number} year
 * @returns {Array}
 */
module.exports = function yearFilter(value, year) {
    let response = [];

    value.map(item => {
        // Pick either the date set in frontmatter or the date eleventy picks from meta info
        const date = new Date(item.data.date ? item.data.date : item.date);
        
        // Parse out the passed year and compare it against this content's full year.
        // Add to the response if there's a match
        if (date.getFullYear() === parseInt(year, 10)) {
            response.push(item);
        }
    });

    return response;
};