demo/typescript.js
function utf16EncodeAsString(codePoint) {
            ts.Debug.assert(0x0 <= codePoint && codePoint <= 0x10FFFF);
            if (codePoint <= 65535) {
                return String.fromCharCode(codePoint);
            }
            var codeUnit1 = Math.floor((codePoint - 65536) / 1024) + 0xD800;
            var codeUnit2 = ((codePoint - 65536) % 1024) + 0xDC00;
            return String.fromCharCode(codeUnit1, codeUnit2);
        }
function utf16EncodeAsString(codePoint) {
            ts.Debug.assert(0x0 <= codePoint && codePoint <= 0x10FFFF);
            if (codePoint <= 65535) {
                return String.fromCharCode(codePoint);
            }
            var codeUnit1 = Math.floor((codePoint - 65536) / 1024) + 0xD800;
            var codeUnit2 = ((codePoint - 65536) % 1024) + 0xDC00;
            return String.fromCharCode(codeUnit1, codeUnit2);
        }
Derived from the 10.1.1 UTF16Encoding of the ES6 Spec.