fussing with UTF-16 surrogate encoding in Javascript

surrogate.js

// see http://en.wikipedia.org/wiki/UTF-16#Code_points_U.2B0000_to_U.2BD7FF_and_U.2BE000_to_U.2BFFFF
function escapeBMP(n) { 
    return '\\u'+('0000'+n.toString(16)).slice(-4); 
}

// see http://en.wikipedia.org/wiki/UTF-16#Code_points_U.2B10000_to_U.2B10FFFF
function makeSurrogate(n) {
    var a = n - 0x10000;
    var highBits = a >> 10;
    var lowBits = a & 0x3FF;
    var leadSurrogate = 0xD800 + highBits;
    var trailSurrogate = 0xDC00 + lowBits;
    return [ leadSurrogate, trailSurrogate ];
}

function escapeSurrogate(n) {
    return makeSurrogate(n).map(escapeBMP).join('');
}

function escapeUTF16(n) {
    if (n >= 0x10000 && <= 0x10FFFF) {
        // code point is in a higher plane and must be encoded as a surrogate pair
        return escapeSurrogate(n);
    }
    else { // if ((n >= 0x0000 && n <= 0xD7FF) || (n >= U+E000 && n <= U+FFFF)) {
        // code point is in the Basic Multilingual Plane
        return escapeBMP(n);
    }
}

var roseEscaped = escapeUTF16(0x1F339);
assert(JSON.parse('"'+roseEscaped+'"') == "\ud83c\udf39")