awesome-string/src/chop/grapheme_at.js

The ultimate JavaScript string library

import coerceToNumber from 'helper/number/coerce_to_number';
import coerceToString from 'helper/string/coerce_to_string';
import nanDefault from 'helper/number/nan_default';
import { REGEXP_UNICODE_CHARACTER } from 'helper/reg_exp/const';

/**
 * Get a grapheme from `subject` at specified `position` taking care of
 * <a href="https://rainsoft.io/what-every-javascript-developer-should-know-about-unicode/#24surrogatepairs">surrogate pairs</a> and
 * <a href="https://rainsoft.io/what-every-javascript-developer-should-know-about-unicode/#25combiningmarks">combining marks</a>.
 *
 * @function graphemeAt
 * @static
 * @since 1.0.0
 * @memberOf Chop
 * @param  {string} [subject=''] The string to extract from.
 * @param  {number} position The position to get the grapheme.
 * @return {string} Returns the grapheme at specified position.
 * @example
 * as.graphemeAt('\uD835\uDC00\uD835\uDC01', 0); // or '𝐀𝐁'
 * // => 'A'
 *
 * as.graphemeAt('cafe\u0301', 3); // or 'café'
 * // => 'é'
 */
export default function graphemeAt(subject, position) {
  const subjectString = coerceToString(subject);
  let positionNumber = coerceToNumber(position);
  let  graphemeMatch;
  let graphemeMatchIndex = 0;
  positionNumber = nanDefault(positionNumber, 0);
  while ((graphemeMatch = REGEXP_UNICODE_CHARACTER.exec(subjectString)) !== null) {
    if (graphemeMatchIndex === positionNumber) {
      REGEXP_UNICODE_CHARACTER.lastIndex = 0;
      return graphemeMatch[0];
    }
    graphemeMatchIndex++;
  }
  return '';
}