JavaScript and the undefined, Infinity, and NaN “keywords”

Consider the following JavaScript program: what do you think it does?

print("undefined before: " + undefined);
undefined = 17;
print("undefined after:  " + undefined);

The above program will print this output:

undefined before: undefined
undefined after:  17

Surely you can’t be serious!

A sane person might think that this program isn’t even a program. Doesn’t undefined always refer to the primitive value undefined? After all, this “program” isn’t one, nor would be the same one for true or false, mutatis mutandis:

print("null before: " + null);
null = 17; // !!! NullLiteral is not a LeftHandSideExpression
print("null after:  " + null);

I am serious…and don’t call me Shirley

Curiously, the program that assigns to undefined is a valid JavaScript program, but programs that assign to null, true, and false are not. Why not? The latter are all keywords with intrinsic meaning within the language; undefined, on the other hand, is just a normal property of the global object. According to ECMA-262 3rd edition, if you assign a different value to undefined, that different value becomes the new value of undefined.

This is a clear botch in ES3. undefined should have been a keyword in JavaScript from the beginning; similarly, the global properties Infinity and NaN probably should have been keywords as well (or perhaps the properties should not have existed, given that Math.Infinity and Math.NaN exist and are immutable). ECMA-262 5th edition doesn’t quite go so far as to change these three properties into keywords due to backwards compatibility concerns (making that change would be guaranteed to break any programs that even tried to assign to those names, regardless whether the program relied on that assignment for correctness). Instead, it changes these properties to be read-only, in the same way that the various numeric properties on the Math object are read-only. Assigning to these properties in ES5 won’t do anything (unless you opt into strict mode, in which case a TypeError exception will be thrown after we fix bug 537873), but at least it won’t definitely and completely break existing programs that relied on this.

We’ve made this change in SpiderMonkey, and it is now in trunk builds of Firefox, slated for the eventual Firefox 3.7 release. Download a nightly build from nightly.mozilla.org and test out the change for yourself (use the profile manager if you want to keep your current Firefox settings and install untouched). This change should have no effect on the vast, vast majority of web developers who don’t try to change the values of these properties; as for the [civility and my religion require I redact this description] developers who did change the value of the global undefined, NaN, or Infinity properties, well…you had it coming.

The bottom line

The global properties undefined, Infinity, and NaN will be read-only and immutable in Firefox 3.7. Assigning to these properties will do nothing (except in strict mode where a TypeError exception will be thrown once we fix a bug) rather than changing their values. This shouldn’t break the vast, vast, vast majority of scripts out there — but there’s no way to guarantee it will break no one, so we think it’s worth announcing this backwards-incompatible change as proactively as possible.