JavaScript strings - a followup

12 May 2006

Having played around with the JavaScript string type some more, I think I understand why it acts as it does. I'm a Perl monkey normally, so I'm not used to the concept of immutable strings, but JavaScript strings are immutable. Playing with the === operator (approximately, 'is this the same object') gives:

js> "a" === "a";
true
js> "a" + "b" === "ab";
true
js> "ab".replace(/./, "c") === "cb";
true

but

js> new String("a") === new String("a");
false

If strings were to magically upgrade themselves to objects, they'd change behaviour - previously equivalent strings would suddenly not be equivalent. Likewise, suppose this worked:

var a = "string";
var b = "string";
a === b; # true
a.foo = 1;

Shoud a still be equivalent to b? If not, a clearly isn't immutable, as we've changed it. But if it is, then we've chanaged b at a distance - it's grown a foo attribute.

Still all very annoying, of course, but I understand why now.