Can the JS array length property ever return a negative value?

Question

Would there ever be an instance when the JavaScript array length property returns a negative value? I'm assuming the answer is no, but I was wondering if there would ever be a need to account for negative values when comparing the length of an array in an if statement, for example.

var x = y.length; if (x === 0) { return false; } else if (x > 0) { return true; } else alert("error"); // is this necessary? }

I would think it would definitely never be negative. But could it be undefined? — xdhmoore
– xdhmoore, Commented Sep 3, 2015 at 22:16
@xdhmoore no, it can only be a natural number (0 or greater). — Pointy
– Pointy, Commented Sep 3, 2015 at 22:20
Can you think of an example of how the .length could possibly be negative? — nnnnnn
– nnnnnn, Commented Sep 3, 2015 at 22:21
How certain are you that y is an array? It could be some other kind of object, with or without a .length property. — StriplingWarrior
– StriplingWarrior, Commented Sep 3, 2015 at 22:21

Michael · Accepted Answer · 2019-06-07 13:31:53Z

27

No.

The spec for the length property says:

The length property of this Array object is a data property whose value is always numerically greater than the name of every deletable property whose name is an array index.

There cannot be -1 properties.

Also, and more explicitly, the spec for Array says:

Every Array object has a length property whose value is always a nonnegative integer less than 2³².

Update: Answer still holds for ES2020

edited Jun 7, 2019 at 13:31

Michael

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answered Sep 3, 2015 at 22:16

josh3736

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7 Comments

Uncle Slug Over a year ago

I had read on w3schools: "Return Value: A Number, representing the number of elements in the array object", which isn't as explicit as your source. This is what lead to my question.

RobG Over a year ago

@UncleSlug—w3schools is a very ordinary site, do not trust it.

RobG Over a year ago

BTW, the current standard is ECMA-262 ed 6 aka ECMAScript 2015. ;-)

cybersam Over a year ago

I am not quibbling with your fine answer, but I want to point out that the first quoted sentence from the spec is shockingly vague. (1) It does not state what the length value is supposed to be if the Array object is empty (that is, if it has no deletable property). What value is numerically greater than something that does not exist? (2) It does not state that (for non-empty Arrays) the length must be "numerically 1 more than the highest deletable property whose name is an array index". It just states that length must be higher than any index.

RobG Over a year ago

@cybersam—unfortunately, it's not all in one place. There is also "Every Array object has a length property whose value is always a nonnegative integer less than 2^32." in ECMASCript 2015 §9.4.2. And yes, the length isn't necessarily always 1 greater, it's just at least one greater.

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potatopeelings · Accepted Answer · 2015-09-03 22:21:32Z

8

Not normally (as other answers have pointed out), but an object that is an instance of an array can have a negative property called length

var b = Object.create([]); b.length = -1; alert(b instanceof Array) alert(b.length);

answered Sep 3, 2015 at 22:21

potatopeelings

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3 Comments

josh3736 Over a year ago

Although, Array.isArray(b) == false

RobG Over a year ago

This highlights a weakness of instanceof that has been long known and why it's not used to test whether an object is an instance of something. ;-)

potatopeelings Over a year ago

yes for both comments. serves as a reminder that the nicer looking check is not always the best :-)

aprok · Accepted Answer · 2015-09-03 22:16:23Z

No, it can't. Moreover, if you try doing this arr.length = -1 it will throw you an exception

sdgluck · Accepted Answer · 2015-09-03 22:16:55Z

No it cannot. MDN says:.

The value of the length property is an integer with a positive sign and a value less than 2 to the 32nd power (232).

Paulo · Accepted Answer · 2015-09-03 22:30:54Z

As said here on Mozilla Developer site (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/length), "the length property represents an unsigned, 32-bit integer that specifies the number of elements in an array.". As you can see, it's an unsigned integer of 32 bits. If it's unsigned, then it cannot be negative.

Check again, clear your cache. Latest update was, oh, 30 seconds before my comment. ;-)

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