esnext.string.is-well-formed.js

'use strict';
var $ = require('../internals/export');
var uncurryThis = require('../internals/function-uncurry-this');
var requireObjectCoercible = require('../internals/require-object-coercible');
var toString = require('../internals/to-string');

var charCodeAt = uncurryThis(''.charCodeAt);

// `String.prototype.isWellFormed` method
// https://github.com/tc39/proposal-is-usv-string
$({ target: 'String', proto: true }, {
  isWellFormed: function isWellFormed() {
    var S = toString(requireObjectCoercible(this));
    var length = S.length;
    for (var i = 0; i < length; i++) {
      var charCode = charCodeAt(S, i);
      // single UTF-16 code unit
      if ((charCode & 0xF800) != 0xD800) continue;
      // unpaired surrogate
      if (charCode >= 0xDC00 || ++i >= length || (charCodeAt(S, i) & 0xFC00) != 0xDC00) return false;
    } return true;
  }
});