To get a copy of a string str without its first and last character :

const newString = str.substring(1, str.length-1)

So :

const str = `"foo"`
console.log(str.substring(1, str.length-1)) // output foo

You may use .replace()

src.replace(/['"]+/g, '')

So in your case

async function watchAnime(episode_id) {
   res = await axios.get(`https://www1.gogoanime.cm/${episode_id}`)
   const body = await res.data;
   $ = cheerio.load(body)

   let src = $('div.play-video > iframe').attr('src')
   let iframe_result = src.replace(/['"]+/g, '')

   return await (iframe_result)
}

You can try use .replace function like this :

const string = `i am' abc `;

console.log(string.replace("'", ''))

console : i am abc

you can read more about is here