How to use five digit long Unicode characters in JavaScript

In the MDN documentation for fromCharCode, they note that javascript will only naturally handle characters up to 0xFFFF. However, they also have an implementation of a fixed method for fromCharCode that may do what you want (reproduced below):

function fixedFromCharCode (codePt) {
    if (codePt > 0xFFFF) {
        codePt -= 0x10000;
        return String.fromCharCode(0xD800 + (codePt >> 10), 0xDC00 + (codePt & 0x3FF));
    }
    else {
        return String.fromCharCode(codePt);
    }
}

foo = fixedFromCharCode(0x1D15D);

Try putting the unicode between curly braces: '\u{1D15D}'.