1// Copyright (C) 2023 The Android Open Source Project 2// 3// Licensed under the Apache License, Version 2.0 (the "License"); 4// you may not use this file except in compliance with the License. 5// You may obtain a copy of the License at 6// 7// http://www.apache.org/licenses/LICENSE-2.0 8// 9// Unless required by applicable law or agreed to in writing, software 10// distributed under the License is distributed on an "AS IS" BASIS, 11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12// See the License for the specific language governing permissions and 13// limitations under the License. 14 15// It's common to want to have a class depending on a boolean flag, in which 16// case we use `flag && className` which evaluates to either false or a string, 17// which is why false is included in definition of ArgType. 18type ArgType = string | false | undefined | ArgType[]; 19 20// Join class names together into valid HTML class attributes 21// Falsy elements are ignored 22// Nested arrays are flattened 23// If all elements are falsy, returns undefined 24export function classNames(...args: ArgType[]): string | undefined { 25 const filtered = args.flat().filter((x) => x); 26 if (filtered.length === 0) { 27 return undefined; 28 } else { 29 return filtered.join(' '); 30 } 31} 32