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ECMAScript spec abstract operations.

There is a newer version: 1.22.3

'use strict';

var GetIntrinsic = require('get-intrinsic');

var $TypeError = GetIntrinsic('%TypeError%');

var callBound = require('call-bind/callBound');

var $charCodeAt = callBound('String.prototype.charCodeAt');
var $numberToString = callBound('Number.prototype.toString');
var $toLowerCase = callBound('String.prototype.toLowerCase');
var $strSlice = callBound('String.prototype.slice');

// https://262.ecma-international.org/9.0/#sec-unicodeescape

module.exports = function UnicodeEscape(C) {
	if (typeof C !== 'string' || C.length !== 1) {
		throw new $TypeError('Assertion failed: `C` must be a single code unit');
	}
	var n = $charCodeAt(C, 0);
	if (n > 0xFFFF) {
		throw new $TypeError('`Assertion failed: numeric value of `C` must be <= 0xFFFF');
	}

	return '\\u' + $strSlice('0000' + $toLowerCase($numberToString(n, 16)), -4);
};