Better undefined gzip compression

[m_gol]

We can compress better if we tell uglifyjs to not mangle undefined, as suggested by dcherman.

----- ORIGINAL TEXT -----
According to ES5.1, undefined is immutable. All supported in 2.0 browsers besides Android <4.1 (including iOS 5.1 and VERY old Safari) adhere to that.

Removing undefined from the main IIFE parameters saves 62 bytes in the gzipped minified file.

I'm putting it here for a discussion. If we decided to go for that, we also have to decide what to do about 1.x

[Rick Waldron]

+1

[Timmy Willison]

Even Android?

[m_gol]

@timmywil, please read my report again, I covered Android.

[Timmy Willison]

Oh, sorry. I don't even know which Android browsers we want to support.

[m_gol]

I guess >= 2.3 currently, unfortunately. :( Anyway, I doubt anyone writes sites only for Android <4.1 so relying on the ability to redefine undefined wouldn't work.

[m_gol]

IE seems to have mutable undefined, after all. I used kangax tables for desktop browsers:
​http://kangax.github.io/es5-compat-table/
but they seem to be wrong about IE9.

Anyway, the spec must be more complicated than what I anticipaded. The following code provides the same results in Chrome, Firefox, Opera & IE10 (IE9 results in [0, 1, ..., 7]):

(function ( undefined ) {
	console.log('0: ' + undefined);
})( 0 ); // => 0

(function ( undefined ) {
	'use strict';
	console.log('1: ' + undefined);
})( 1 ); // => 1

(function () {
	undefined = 2;
	console.log('2: ' + undefined);
})(); // => 2

(function () {
	'use strict';
	try {
		undefined = 3;
		console.log('3: ' + undefined);
	} catch (e) {
		console.log('3: ' + e);
	}
})(); // => TypeError

(function () {
	var undefined = 4;
	console.log('4: ' + undefined);
})(); // => 4

(function () {
	'use strict';
	var undefined = 5;
	console.log('5: ' + undefined);
})(); // => 5

var undefined = 6;
(function () {
	console.log('6: ' + undefined);
})(); // => undefined

var undefined = 7;
(function () {
	'use strict';
	console.log('7: ' + undefined);
})(); // => undefined

[m_gol]

On jsFiddle every browser behaves like IE9, though...

​http://jsfiddle.net/m_gol/3KrZW/2/

[dmethvin]

On IRC, dcherman suggested just telling uglify to not compress undefined, which he says saves 40-something bytes. Not quite as much saved, but no downside.

[m_gol]

Yeah... I didn't suspect ES5's meaning of undefined is that you can't do undefined = 'a' but you can do var undefined = 'a'. Sooo helpful...

[m_gol]

Pull request: ​https://github.com/jquery/jquery/pull/1239

[Michał Gołębiowski]

Tell uglifyjs to not mangle undefined; saves 44 bytes. Fixes #13759. Close gh-1239.

Changeset: 8576e24bfe3fce886bcb63ebc24ee16b3fa1a6a1

[m_gol]

BTW, orkel was right, this change actually increases the gzipped 1.x-master size so it makes sense only on master.