Move getFormatterFunction back into standalone

lib/standalone.js

@@ -6,8 +6,9 @@ const createStylelintResult = require('./createStylelintResult');
 const debug = require('debug')('stylelint:standalone');
 const FileCache = require('./utils/FileCache');
 const filterFilePaths = require('./utils/filterFilePaths');
+const formatters = require('./formatters');
 const fs = require('fs');
-const getFormatterFunction = require('./getFormatterFunction');
+const getFormatterOptionsText = require('./utils/getFormatterOptionsText');
 const globby = require('globby');
 const hash = require('./utils/hash');
 const NoFilesFoundError = require('./utils/noFilesFoundError');
@@ -24,6 +25,7 @@ const writeFileAtomic = require('write-file-atomic');
 /** @typedef {import('stylelint').StylelintStandaloneReturnValue} StylelintStandaloneReturnValue */
 /** @typedef {import('stylelint').StylelintResult} StylelintResult */
 /** @typedef {import('stylelint').Formatter} Formatter */
+/** @typedef {import('stylelint').FormatterIdentifier} FormatterIdentifier */
 
 /**
  * @param {StylelintStandaloneOptions} options
@@ -284,6 +286,31 @@ module.exports = function (options) {
 		});
 };
 
+/**
+ * @param {FormatterIdentifier | undefined} selected
+ * @returns {Formatter}
+ */
+function getFormatterFunction(selected) {
+	/** @type {Formatter} */
+	let formatterFunction;
+
+	if (typeof selected === 'string') {
+		formatterFunction = formatters[selected];
+
+		if (formatterFunction === undefined) {
+			throw new Error(
+				`You must use a valid formatter option: ${getFormatterOptionsText()} or a function`,
+			);
+		}
+	} else if (typeof selected === 'function') {
+		formatterFunction = selected;
+	} else {
+		formatterFunction = formatters.json;
+	}
+
+	return formatterFunction;
+}
+
 /**
  * @param {import('stylelint').StylelintInternalApi} stylelint
  * @param {any} error