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  2. GOV.UK Prototype Filters

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Node.js v18 or later.


To install, in your terminal type:

npm install @x-govuk/govuk-prototype-filters

If you are using version 13 or later of the GOV.UK Prototype Kit, the filters will be immediately available for use in Nunjucks templates.

The plugin can be managed alongside other plugins in your prototype.

Using with earlier versions of the Prototype Kit

If you are using an earlier version of the Prototype Kit, import the filters into /app/filters.js:

+ const prototypeFilters = require('@x-govuk/govuk-prototype-filters');

  module.exports = function (env) {
     * Instantiate object used to store the methods registered as a
     * 'filter' (of the same name) within nunjucks. You can override
     * core filters by creating filter methods of the same name.
     * @type {Object}
-   var filters = {}
+   var filters = prototypeFilters

    // Existing filter
    filters.sayHi = function(name) {
        return 'Hi ' + name + '!'

    return filters

Advanced usage

@x-govuk/govuk-prototype-filters exports an object containing all filter functions.

Using Nunjucks’ addFilter method you can add individual filters to your Nunjucks environment:

const { slugify } = require('@x-govuk/govuk-prototype-filters')
const nunjucks = require('nunjucks')

const nunjucksEnv = nunjucks.configure(['./app/views'])

nunjucksEnv.addFilter("slugify", slugify)