Yet another javascript wat moment

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qqqyqnz · 40 points · Posted at 20:23:11 on June 21, 2016 · (Permalink)

It's not a wat moment, it's a bug. Actual wat moments are language design choices.

BlueShellOP · 47 points · Posted at 06:20:57 on June 22, 2016 · (Permalink)

The real joke is that we're not even sure if it's a bug or a feature of Javascript anymore.

[deleted] · 50 points · Posted at 14:12:47 on June 21, 2016 · (Permalink)*

[deleted]

Laat · 96 points · Posted at 14:33:34 on June 21, 2016 · (Permalink)*

it is a JIT bug in chrome 51

relevant thread over at /r/javascript.

[deleted] · 24 points · Posted at 16:57:42 on June 21, 2016 · (Permalink)

A jitbug you say?

DrScabhands · 1 points · Posted at 13:15:27 on June 22, 2016 · (Permalink)

I thought you were going to link to this

LoveandRockets · -1 points · Posted at 20:19:28 on June 21, 2016 · (Permalink)

Jitbug!

backflipbail · 1 points · Posted at 05:45:45 on June 22, 2016 · (Permalink)

Jitterbug

spambot5546 · 4 points · Posted at 14:52:47 on June 21, 2016 · (Permalink)

The js typeof operator identifies the datatype of a given value, either a literal or the value in a variable. I can do

var i = "a";

console.log(typeof i);

and it'll print "string" because that's a string.

If an variable has not been assigned a memory location it is "null" and, especially in js, cannot be meaningfully said to have a type.

I tried running this in jsfiddle and it considered null to be an object. Using an undefined variable e.g.

var i;

console.log(typeof i);

gave me "undefined", but did so consistently.

pxndxx · 13 points · Posted at 18:19:57 on June 21, 2016 · (Permalink)

But null is an object in JS. in your second example, i is a variable of type undefined, and its value is undefined too.

Variables that haven't been assigned a value in JS are undefined, not null.

OptimisticLockExcept · 1 points · Posted at 17:14:15 on June 24, 2016 · (Permalink)

Actually null is not an object. The ECMAScript specification defines the types in ECMAScript. There is a Null Type and an Object type. (and other types)

The specification also defines that typeof null === "object"

Whether or not this was a bug originaly or not is arguable.

I'd link the spec but i'm on mobile

AquaWolfGuy · 5 points · Posted at 20:41:51 on June 21, 2016 · (Permalink)

Variables without a memory location aren't anything since they don't exist. Variables that have been declared and allocated but not assigned to (as in your second example) have the value undefined. typeof undefined is "undefined". null is not the same as undefined.

Browsing_From_Work · 1 points · Posted at 15:43:46 on June 22, 2016 · (Permalink)

Regardless if null is an object or not, the result of typeof null shouldn't change during execution.

Matizaurus · -6 points · Posted at 14:32:41 on June 21, 2016 · (Permalink)

Doesn't happen on Firefox. User agent stuff?

jonatcer · 32 points · Posted at 15:17:07 on June 21, 2016 · (Permalink)

Because it's not a Javascript issue.

more_downvotes_pls · 8 points · Posted at 14:38:18 on June 21, 2016 · (Permalink)

All return false for me in Chrome 51 on OS X.

Is it always 4917/5083 or is it random? Curious if you get the same with ===

svtguy88 · 8 points · Posted at 15:53:06 on June 21, 2016 · (Permalink)

Appears to be somewhat random, but only from machine to machine. I get 4099 false and 5901 true for both == and ===. I get the same result with multiple runs.

Chrome 51.0.2704.103 on Windows 10. I'm curious what the Linux build does. I'm guessing it mimics OS X's results, but will have to wait till I get home to check (no *nix on the work laptop).

[deleted] · 25 points · Posted at 17:52:12 on June 21, 2016 · (Permalink)

Appears to be somewhat random

someone make a random number generator based on this

Cley_Faye · 2 points · Posted at 18:37:39 on June 27, 2016 · (Permalink)

Last time we used undefined behavior to generate randomness we had to wipe all our ssh keys, so no thanks :|

taar779 · 9 points · Posted at 16:35:50 on June 21, 2016 · (Permalink)

Linux Chrome Version 51.0.2704.103 (64-bit)

for (let i=0; i<10000; i++) { console.log(typeof null == 'undefined'); }
4917 false
5083 true

Same result each time I ran it.

Chromium Version 51.0.2704.84 (64-bit) has the same result as well.

Saveman71 · 3 points · Posted at 16:49:22 on June 21, 2016 · (Permalink)

On Linux, build 51.0.2704.84 (64-bit), same results as well

nice_comment_thanks · 2 points · Posted at 18:19:06 on June 21, 2016 · (Permalink)

I'm on OS X, chrome version 51.0.2704.103 (64-bit), also 4917 false / 5083 true

more_downvotes_pls · 2 points · Posted at 16:45:54 on June 21, 2016 · (Permalink)

Weird. My OS X version is 51.0.2704.103 (64-bit) for what it's worth.

for (let i=0; i<10000; i++) { console.log(typeof null == 'undefined'); }
10000 false

nice_comment_thanks · 2 points · Posted at 18:20:57 on June 21, 2016 · (Permalink)

That's weird! I'm also on OS X (10.11.5), also with Chrome version 51.0.2704.103 (64-bit), but I get 4917 times false and 5083 times true.

more_downvotes_pls · 2 points · Posted at 19:15:11 on June 21, 2016 · (Permalink)

Ah, I got your numbers when I made a new file and just opened it in the browser.

Before I was I had just plugged it into my current project environment which is running through Babel and all sorts of ES6 and React things. I believe Babel is probably compiling scoped var equivalents of let or something so that that explains it.

wpreggae · 2 points · Posted at 16:06:19 on June 21, 2016 · (Permalink)

I get the same false/true count

[deleted] · 1 points · Posted at 22:09:20 on June 21, 2016 · (Permalink)

I'm on Chrome 51.0.2704.103 on Windows 10 too and get the exact same results as you: 4099/5901.

xXAndrew28Xx · 1 points · Posted at 23:50:53 on June 21, 2016 · (Permalink)

I got the exact same result as you. Outdated Chrome (51.0.2704.84 32 bit) on Windows 7 64 bit.

Fs0i · 5 points · Posted at 17:02:29 on June 21, 2016 · (Permalink)

It's a Chrome 51 Windows / Linux JIT-Bug. It's "random" - as soon as V8 JITs the method it replaces null by undefined internally for speed (since they should behave the same), which throws typeof off.

anotherdonald · 1 points · Posted at 17:57:46 on June 21, 2016 · (Permalink)

> var x = undefined
undefined
> x === undefined
true
> x === null
false
> var y = null
undefined
> y === undefined
false
> y === null
true

Fs0i · 5 points · Posted at 18:55:06 on June 21, 2016 · (Permalink)

Sorry if that was a bit unclear: V8 thinks that null / undefined are exchangeable in the function to optimise, when they obviously are not.

Cilph · 37 points · Posted at 14:21:10 on June 21, 2016 · (Permalink)

Am I going to have to start doing this?

var x = 2;
x = 2; // to make sure
doStuff(x);
x = 2; // to make sure x doesn't lose its value

jonatcer · 33 points · Posted at 15:18:58 on June 21, 2016 · (Permalink)

No, because it's a bug in Chrome.

jerkenstine · 2 points · Posted at 23:53:31 on June 21, 2016 · (Permalink)

It's actually fixed in Chrome v53+.

boxingdog · 1 points · Posted at 01:33:28 on June 22, 2016 · (Permalink)

don't want to scare you but js does hoisting

Cilph · 1 points · Posted at 05:31:25 on June 22, 2016 · (Permalink)

Yup...

hanszimmermanx · 7 points · Posted at 20:52:57 on June 21, 2016 · (Permalink)

If anything its a chrome/v8 wat moment but I guess it doesn't matter, the JS hate sheep upvotes anyway.

Tinfoilpain · 3 points · Posted at 00:03:13 on June 22, 2016 · (Permalink)

it's b'aaaaaad

jerkenstine · 1 points · Posted at 23:54:01 on June 21, 2016 · (Permalink)

It's already fixed anyways.

jerkenstine · 3 points · Posted at 23:55:30 on June 21, 2016 · (Permalink)

FYI this is fixed in Chrome 53+.

It'll be fixed in node when node picks up V8 5.1 as well.

coladict · 2 points · Posted at 10:47:42 on June 22, 2016 · (Permalink)

I've never actually used let. Is there any advantage over var? And most importantly, does it work on IE9+?

lukeilsluke · 2 points · Posted at 17:21:06 on June 23, 2016 · (Permalink)

A let stays in the scope of a code block and they're only hoisted to the top of the code block they're declare in. Not a ton of practical use, but there are some situations where you'd rather have them

DAVIDSPZGZ · 1 points · Posted at 08:55:30 on June 23, 2016 · (Permalink)

JavaScript is a full mystery

Miles-za · 1 points · Posted at 00:44:17 on June 24, 2016 · (Permalink)

I'd like to see someone make an RNG out of this.

TheDarkIn1978 · 0 points · Posted at 21:46:27 on June 21, 2016 · (Permalink)*

I actually just read something about this yesterday. Apparently, JavaScript has a long standing bug which outputs "object" instead of "null":

var a = null;    
typeof(a); //"object"

as one of JavaScript's 7 value types:

string, number, boolean, null, undefined, object and symbol

Apparently it has never been fixed, nor will ever be fixed, because of the vast amount of code already on the web that relies on this bug to function properly.

[EDIT]: SOURCE.

erikdesjardins · 4 points · Posted at 23:35:54 on June 21, 2016 · (Permalink)

Did they have a source for that claim?

In all of the ECMAScript specifications back to ES1 in 1997 (ctrl-f "the typeof operator"), it's specified that typeof null is 'object'.

I can't find the original 1995 LiveScript spec, but I don't think it would have been any different.

TheDarkIn1978 · -1 points · Posted at 00:34:06 on June 22, 2016 · (Permalink)*

[EDIT] Read for yourself here.

Ok, so I assume the bug has always been there ever since the first ES1 spec. Why would null return "object" instead of "null"?

var str = "";
console.log(typeof(str));  //"string"

var num = 0;
console.log(typeof(num));  //"number"

var bool = false;
console.log(typeof(bool));  //"boolean"

var n = null;
console.log(typeof(n));  //"object" (why not "null"?)

var un;
console.log(typeof(un));  //"undefined"

var obj = {};
console.log(typeof(obj));  //"object"

var sym = Symbol();
console.log(typeof(sym));  //"symbol"

compteNumero9 · 1 points · Posted at 08:30:23 on June 22, 2016 · (Permalink)

Hu ? This isn't a bug. typeof null is specified to be "object", just like typeof NaN is specified to be "number", which makes sense because null is the "missing" value when you expect an object.

poizan42 · 2 points · Posted at 16:26:30 on June 27, 2016 · (Permalink)

It was probably a bug (or at least "unintentional behavior") in Netscape Navigator 2.0 that later got enshrined into the standard to keep backwards compatibility.

compteNumero9 · 1 points · Posted at 16:31:51 on June 27, 2016 · (Permalink)

In fact you're right. I checked and even Eich thinks it's mostly a bug.

TheDarkIn1978 · 0 points · Posted at 10:50:34 on June 22, 2016 · (Permalink)

Ok, but unlike null, NaN isn't one of JavaScript's 7 value types, so there at least appears to be some inconsistency.

OKB-1 · 0 points · Posted at 16:18:11 on June 22, 2016 · (Permalink)

JavaScript dev protip: Try to avoid ever using typeof.