Function Core.getHtmlOfUrl returns promise. So you need return this promise to a caller.

export function url(externalUrl, language = '', format = 'default', options = {
lengthLow: LENGTH_LOW_DEFAULT,
lengthHigh: LENGTH_HIGH_DEFAULT,
stopwordsLow: STOPWORDS_LOW_DEFAULT,
stopwordsHigh: STOPWORDS_HIGH_DEFAULT,
maxLinkDensity: MAX_LINK_DENSITY_DEFAULT,
maxHeadingDistance: MAX_HEADING_DISTANCE_DEFAULT,
noHeadings: NO_HEADINGS_DEFAULT,
}) {
return Core.getHtmlOfUrl(externalUrl).then((response) => {
const htmlText = response.data;
return rawHtml(htmlText, language, format, options);
}).catch((error) => {
throw error;
});
}

And justext.url(url, language, 'detailed', options).then((data)=>console.log(data));