yamlize() - A simple way to turn JSON objects into YAML strings.

yamlize.js

function yamlize(value, spaces) {
  function recurseYamlize(value, level) {
    if (value == null) {
      return 'null';
    }
  
    var indentation = spacing.repeat(level);
    const typeName = Object.prototype.toString.call(value).slice(8, -1);
  
    if (typeName === 'Array') {
      
      return value.length
        ? value.map(v => {
            return '\n' + indentation + '- ' + recurseYamlize(v, level + 1);
          }).join('')
        : '[]';
    }
    else if (typeName === 'Number' || typeName === 'Boolean') {
      return `${value}`;
    }
    else if (typeName === 'String') {
      // If starts with punctuation or whitespace or ends with ":" or "-" or is
      // the empty string or is surrounded by single or double quotes or ends
      // with a hyphen or a colon or contains a # or a tab or a non-printable
      // character or is one of the following: true, false, null, or a number.
      if (/^[~`!@#$%^&*()\-+={}\[\]\|\\;:<>?,.\/]|^(?:"[^]*"|'[^]*'|true|false|null|\d+(?:\.\d+)?|)$|[\-:]$|[#\t\u0000-\u0008\u000B-\u001F\u007F-\u009F\u2000-\u200F\u2028-\u202F\u205F-\u206F\u3000\uFEFF]/i.test(value)) {
        return JSON.stringify(value);
      }
      // If no leading or trailing spaces and no line breaks.
      if (!/^ |\n| $/.test(value)) {
        return value;
      }
      return '|'
        + (/^\n* +/.test(value) ? spaces : '')
        + (value.endsWith('\n\n') ? '+' : value.endsWith('\n') ? '' : '-')
        + '\n'
        + value.replace(/^(?!$)/gm, indentation + (level === 0 ? spacing : '')).replace(/\n$/, '');
    }
    else if (typeName === 'Date') {
        return value.toJSON();
    }
    value = Object(value);
    const keys = Object.keys(value);
    if (!keys.length) {
      return '{}';
    }
    return keys.map(k => {
      const v = value[k];
      // If starts with punctuation or whitespace or ends with ":" or "-" or is
      // the empty string or is surrounded by single or double quotes or ends
      // with a hyphen or a colon or contains a # or a tab or a newline
      // character or a non-printable character or is one of the following:
      // true, false, null, or a number.
      if (/^[~`!@#$%^&*()\-+={}\[\]\|\\;:<>?,.\/\s]|^(?:"[^]*"|'[^]*'|true|false|null|\d+(?:\.\d+)?|)$|[\-:\s]$|[#\t\n\u0000-\u0008\u000B-\u001F\u007F-\u009F\u2000-\u200F\u2028-\u202F\u205F-\u206F\u3000\uFEFF]/i.test(k)) {
        k = JSON.stringify(k);
      }
      return '\n' + indentation + k + ': ' + recurseYamlize(v, level + 1);
    }).join('');
  }

  const spacing = ' '.repeat(spaces = spaces || 2);

  return recurseYamlize(value, 0).trimLeft();
}

~io-app.js

function runIOApp(input, spaces) {
  return yamlize(JSON.parse(input), +spaces || 2);
}

~io-config.jsonc

{
  // Where the JS code lies
  "files": ["yamlize.js", "~io-app.js"],
  // Name of the function responsable for transforming the input into the output.
  "transform": "runIOApp",
  // Arguments that will be passed to the function by name.
  "params": [
    {
      "name": "spaces",
      "label": "Number of spaces to indent with:",
      "type": "number",
      "min": 1,
      "max": 8,
      "step": 1,
      "value": 2
    }
  ],
  // Input setup
  "input": { "language": "json" },
  // Output setup
  "output": { "language": "yaml" },
  // Type of YourJS IO-App
  "type": "split"
}