Internal String Storage In JavaScript?

Does anyone have a reputable reference about internal string storage in
JavaScript? (for some particular implementation I mean).

Say having 1,048,576 characters long string from the geometric
progression:

function generateLargeString() {
var s = 'a'
for (var i=1; i<21; ++i) {
s = s.concat(s);
}
return s;
}

- the internal size should be 2 mebibytes and not 1 (?) if strings are
indeed stored as Unicode 16-bit. From the other hand it would be
tempting for an engine developer do not spend extra bytes on ASCII
chars...

So does anyone know of any documented engine optimizations on the
matter? Would be expected on some engine to have the string from above
twice smaller than say

function generateLargeString() {
// 1200 ETHIOPIC SYLLABLE HA
var s = String.fromCharCode(0x1200);
for (var i=1; i<21; ++i) {
s = s.concat(s);
}
return s;
}