Function that accepts clear text with Telegram Core API styled text message entities and converts it to HTML.

telegram-mtproto-entities-to-html.ts

import { Api } from 'telegram'
// gram.js format

const insert = (text: string, start: number, substring: string) => {
  return text.substring(0, start) + substring + text.substring(start, text.length)
}

export function styleEntitiesToHTML(text: string, entities: Api.TypeMessageEntity[]): string {
  let html = text
  const tags = {
    MessageEntityUrl: 'a',
    MessageEntityBold: 'b',
    MessageEntityItalic: 'i',
    MessageEntityCode: 'pre',
    MesageEntityPre: 'pre',
    MessageEntityBlockquote: 'pre',
    MessageEntitySpoiler: 'pre',
    MessageEntityTextUrl: 'a',
    MessageEntityUnderline: 'u',
    MessageEntityStrike: 'strikethrough',
  }
  let offset = 0
  for(const entity of entities) {
    if(!Object.hasOwn(tags, entity.className)) continue

    // @ts-expect-error Line above checks for className key
    const tag = tags[entity.className]

    const attributes = tag === 'a' ? ` href='${entity['url']}'` : ''

    const htmlOpenTag = `<${tag}${attributes}>`
    html = insert(html, entity.offset + offset, htmlOpenTag)
    offset += htmlOpenTag.length
    
    const htmlCloseTag = `</${tag}>`
    html = insert(html, entity.offset + offset + entity.length, htmlCloseTag)
    offset += htmlCloseTag.length
  }
  return html
}