Why does ",,," == Array(4) in JavaScript?

Question

Boot up your interpreter/console and try the comparison:

> ",,," == Array(4)
True

Why? At first I thought maybe since you could think of ",,," as an array of four characters with a '\0' terminating slice, that might be why, but

> "..." == Array(4)

Returns "False". So... why? I know it's some idiosyncratic bit of duck typing in JavaScript, but I am just curious what underlines this behavior. I gleaned this from Zed Shaw's excellent presentation here, btw.

Few languages apart from C use zero-termination in a way visible to the programmer. — Joey
– Joey, Commented Jun 5, 2012 at 21:39
@SomeKittens Zed Shaw mentions this explicitly in the video I linked to in my question (as a criticism of Javascript). Cheers! — ZenLikeThat
– ZenLikeThat, Commented Jun 6, 2012 at 2:23
@SomeKittens This is also mentioned in the (pretty well known) "wat" talk, showing some quirks in Ruby and JavaScript destroyallsoftware.com/talks/wat — Cronco
– Cronco, Commented Jun 6, 2012 at 11:26
This is one of many good reasons to always use === instead of ==. — wprl
– wprl, Commented Jun 12, 2012 at 19:52

Felix Kling · Accepted Answer · 2012-06-05 21:51:16Z

176

Because the right hand operand is converted to a string and the string representation of Array(4) is ,,,:

> Array(4).toString()
  ",,,"

If you use the array constructor function and pass a number, it sets the length of the array to that number. So you can say you have four empty indexes (same as [,,,]) and the default string representation of arrays is a comma-separated list of its elements:

> ['a','b','c'].toString()
  "a,b,c"

How the comparison works is described in section 11.9.3 of the specification. There you will see (x == y):

8. If Type(x) is either String or Number and Type(y) is Object,
return the result of the comparison x == ToPrimitive(y).

_{(arrays are objects in JavaScript)}

and if you follow the ToPrimitive method you will eventually find that it it calls toString.

edited Jun 5, 2012 at 21:51

answered Jun 5, 2012 at 21:40

Felix Kling

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2 Comments

user149341 Over a year ago

Which, in turn, is because arrays stringify as if they were using Array.join(",").

matcheek Over a year ago

You have to be bloody patient to program in javascript

Peter Mortensen · Accepted Answer · 2023-08-18 22:03:10Z

32

Try using ===. When using == in JavaScript, it will attempt to cast the variables, thus leading to issues like this one. The console is casting Array(4) to the string representation (i.e., Array(4).toString), which is ",,,". The reason the commas are there is that the .toString() function adds them to separate items in an array.

See the snippet below:

document.write( Array(4).toString() );

edited Aug 18, 2023 at 22:03

Peter Mortensen

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answered Jun 5, 2012 at 21:40

SomeKittens

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Comments

Peter Mortensen · Accepted Answer · 2023-08-18 22:01:21Z

31

Internally, it’s going:

",,," == Array(4).toString()

edited Aug 18, 2023 at 22:01

Peter Mortensen

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answered Jun 5, 2012 at 21:40

Jason Kulatunga

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Keith · Accepted Answer · 2012-06-05 22:31:22Z

18

This is because Array(4) initialises an array of 4 empty values, an == implicitly converts, so:

 ",,," == Array(4)

 ",,," == Array(4).toString()

 ",,," == ["", "", "", ""] // note 3 commas for 4 values

 ",,," == ["", "", "", ""].toString()

Are all similar.

== does implicit type conversions before comparing the values, which can result in unpredictable results. Use === to check the type and the value.

answered Jun 5, 2012 at 22:31

Keith

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Comments

Russell Borogove · Accepted Answer · 2012-06-05 21:41:00Z

5

Comparing an Array to a string coerces the Array to a string before doing the comparison. Coercing an empty 4-element Array to a string yields that exact string.

answered Jun 5, 2012 at 21:41

Russell Borogove

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Peter Mortensen · Accepted Answer · 2023-08-18 21:59:45Z

4

I first thought it was something with the "prototype"... But after a little investigation, I reached a sad conclusion...

Apparently it is an internal and more obscure JavaScript thing with not much logic...

Just try

Array(4)==Array(4)

and no coercion on types also...

Array(4)===Array(4)

and you'll get FALSE.

You know that null==null, null===null and even undefined==undefined and undefined===undefined returns TRUE... so... it's a bit obscure...

Array(4)==[,,,] should also be true.

edited Aug 18, 2023 at 21:59

Peter Mortensen

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answered Jun 7, 2012 at 21:56

ZEE

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2 Comments

devsathish Over a year ago

ZEE, Array(4)==[,,,] wont be true. If we compare object with primitive, then the object will be converted into primitive. Thats the reason it calls toString().

ZEE Over a year ago

array(x) should be the address of the constructor... anyway, in a system (dont bother what kind of system), <identity_X>===<identity_X> sould always be true!

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