czech-inflection is JavaScript library for inflecting Czech words. It is a port of PHP inflection library.
npm i czech-inflection
import inflect from 'czech-inflection'
console.log(inflect({ word: 'Tomáš', grammarCase: 3, animate: true }))
// Tomášovi
console.log(inflect({ word: 'Tomáš', grammarCase: 3, animate: true, plural: true }))
// Tomášům
console.log(inflect({ word: 'Honza', grammarCase: 3, animate: true, gender: 'm' }))
// It's necessary to specify gender for some names/nicknames to inflect them correctly
// Honzovi
console.log(inflect({ word: 'hrad', grammarCase: 6 }))
// hradě
console.log(inflect({ word: 'hrad', grammarCase: 6, plural: true }))
// hradech