//helpers.js
module.exports = {
  truncate: (str, len) => {
    //code
    return str;
  },

  stripTags: input => input.replace(/<(?:.|n)*?>/gm, ''),

  formatDate: (date, format) => moment(date).format(format),

  select: function (selected, options) {
    return options.fn(this).replace(new RegExp(` value="${selected}"`), '$&selected="selected"').replace(new RegExp(`>${selected}</option>`), 'selected="selected"$&');
  },
};

//sets application locals so I can call these functions from a pug template
app.locals = {
  truncate,
  stripTags,
  formatDate,
  select,
};

#{formatDate(story.date, 'MMMM Do YYYY')}

select(name='status' id='status')
  #{select(story.status)}
    option(value='Public' selected) Public
    option(value='Private') Private
    option(value='Unpublished') Unpublished
label(for='status') Status

Cannot read property 'fn' of undefined

<select name ='status' id='status'>
        {{#select story.status}}
          <option value='Public' selected> Public </option
          <option value='Private'> Private </option
          <option value='Unpublished'> Unpublished </option>
        {{#/select}}

Warning
Filters are rendered at compile time. This makes them fast, but it also means that they cannot support dynamic content or options.

By default, compilation in the browser does not have access to JSTransformer-based filters, unless the JSTransformer modules are explicitly packed and made available through a CommonJS platform (such as Browserify or Webpack). In fact, the page you are reading right now uses Browserify to make the filters available in the browser.

Templates pre-compiled on the server do not have this limitation.