node_modules/es5-ext/string/#/at.js - master - eb/Surf

// Based on: https://github.com/mathiasbynens/String.prototype.at
// Thanks @mathiasbynens !

"use strict";

var toInteger  = require("../../number/to-integer")
  , validValue = require("../../object/valid-value");

module.exports = function (pos) {
	var str = String(validValue(this)), size = str.length, cuFirst, cuSecond, nextPos, len;
	pos = toInteger(pos);

	// Account for out-of-bounds indices
	// The odd lower bound is because the ToInteger operation is
	// going to round `n` to `0` for `-1 < n <= 0`.
	if (pos <= -1 || pos >= size) return "";

	// Second half of `ToInteger`
	// eslint-disable-next-line no-bitwise
	pos |= 0;
	// Get the first code unit and code unit value
	cuFirst = str.charCodeAt(pos);
	nextPos = pos + 1;
	len = 1;
	if (
		// Check if it’s the start of a surrogate pair
		cuFirst >= 0xd800 &&
		cuFirst <= 0xdbff && // High surrogate
		size > nextPos // There is a next code unit
	) {
		cuSecond = str.charCodeAt(nextPos);
		if (cuSecond >= 0xdc00 && cuSecond <= 0xdfff) len = 2; // Low surrogate
	}
	return str.slice(pos, pos + len);
};