voca/code_point_at.js

The ultimate JavaScript string library

'use strict';

require('./internal/is_nil.js');
require('./is_string.js');
var coerce_to_string = require('./internal/coerce_to_string.js');
var coerce_to_number = require('./internal/coerce_to_number.js');
var nan_default = require('./internal/nan_default.js');

var HIGH_SURROGATE_START = 0xd800;
var HIGH_SURROGATE_END = 0xdbff;
var LOW_SURROGATE_START = 0xdc00;
var LOW_SURROGATE_END = 0xdfff;
/**
 * Checks if `codePoint` is a high-surrogate number from range 0xD800 to 0xDBFF.
 *
 * @ignore
 * @param {number} codePoint The code point number to be verified
 * @return {boolean} Returns a boolean whether `codePoint` is a high-surrogate number.
 */

function isHighSurrogate(codePoint) {
  return codePoint >= HIGH_SURROGATE_START && codePoint <= HIGH_SURROGATE_END;
}
/**
 * Checks if `codePoint` is a low-surrogate number from range 0xDC00 to 0xDFFF.
 *
 * @ignore
 * @param {number} codePoint The code point number to be verified
 * @return {boolean} Returns a boolean whether `codePoint` is a low-surrogate number.
 */

function isLowSurrogate(codePoint) {
  return codePoint >= LOW_SURROGATE_START && codePoint <= LOW_SURROGATE_END;
}
/**
 * Get the astral code point number based on surrogate pair numbers.
 *
 * @ignore
 * @param {number} highSurrogate The high-surrogate code point number.
 * @param {number} lowSurrogate The low-surrogate code point number.
 * @return {number} Returns the astral symbol number.
 */

function getAstralNumberFromSurrogatePair(highSurrogate, lowSurrogate) {
  return (highSurrogate - HIGH_SURROGATE_START) * 0x400 + lowSurrogate - LOW_SURROGATE_START + 0x10000;
}

/**
 * Get the Unicode code point value of the character at `position`. <br/>
 * If a valid UTF-16 <a href="https://rainsoft.io/what-every-javascript-developer-should-know-about-unicode/#24surrogatepairs">
 * surrogate pair</a> starts at `position`, the
 * <a href="https://rainsoft.io/what-every-javascript-developer-should-know-about-unicode/#astralplanes">astral code point</a>
 * value at `position` is returned.
 *
 * @function codePointAt
 * @static
 * @since 1.0.0
 * @memberOf Chop
 * @param  {string} [subject=''] The string to extract from.
 * @param  {number} position The position to get the code point number.
 * @return {number} Returns a non-negative number less than or equal to `0x10FFFF`.
 * @example
 * v.codePointAt('rain', 1);
 * // => 97, or 0x0061
 *
 * v.codePointAt('\uD83D\uDE00 is smile', 0); // or '😀 is smile'
 * // => 128512, or 0x1F600
 */

function codePointAt(subject, position) {
  var subjectString = coerce_to_string.coerceToString(subject);
  var subjectStringLength = subjectString.length;
  var positionNumber = coerce_to_number.coerceToNumber(position);
  positionNumber = nan_default.nanDefault(positionNumber, 0);

  if (positionNumber < 0 || positionNumber >= subjectStringLength) {
    return undefined;
  }

  var firstCodePoint = subjectString.charCodeAt(positionNumber);
  var secondCodePoint;

  if (isHighSurrogate(firstCodePoint) && subjectStringLength > positionNumber + 1) {
    secondCodePoint = subjectString.charCodeAt(positionNumber + 1);

    if (isLowSurrogate(secondCodePoint)) {
      return getAstralNumberFromSurrogatePair(firstCodePoint, secondCodePoint);
    }
  }

  return firstCodePoint;
}

module.exports = codePointAt;