Twig functions: add a 'yaml' filter

doc/TwigJS.md

@@ -80,6 +80,7 @@ Extra filters:
 * filter `wikidataEntity`: returns the wikidata entity in structured form (or `null` if the entity is not cached or `false` if it does not exist). Example: https://www.wikidata.org/wiki/Special:EntityData/Q42.json
 * filter `json_pp`: JSON pretty print the object. As parameter to the filter, the following options can be passed:
   * `indent`: indentation (default: 2)
+* filter `yaml`: YAML pretty print the object. As options the filter, all options to [yaml.dump of js-yaml](https://github.com/nodeca/js-yaml#dump-object---options-) can be used.
 
 Notes:
 * Variables will automatically be HTML escaped, unless the filter raw is used, e.g.: `{{ tags.name|raw }}`

src/twigFunctions.js

@@ -5,6 +5,7 @@ var osmParseDate = require('openstreetmap-date-parser')
 var osmFormatDate = require('openstreetmap-date-format')
 const natsort = require('natsort').default
 const md5 = require('md5')
+const yaml = require('js-yaml')
 
 var md5cache = {}
 
@@ -148,3 +149,11 @@ OverpassLayer.twig.extendFilter('json_pp', function (value, param) {
 
   return JSON.stringify(value, null, 'indent' in options ? ' '.repeat(options.indent) : '  ')
 })
+OverpassLayer.twig.extendFilter('yaml', function (value, param) {
+  const options = param[0] || {}
+
+  value = JSON.parse(JSON.stringify(value))
+  delete value._keys // remove TwigJS artefact
+
+  return yaml.dump(value, options)
+})