How does JavaScript determine if an array index is integer?

Question

It seems that JavaScript's array indexes are actually all strings, so a[0] is the same as a['0'] while a[1.0] is not a[1] but a['1.0']. But at the same time, array has a length property; it will be updated automatically when you modify value of integer keys. So how does JavaScript know the key is an integer and it needs to change length? If I do:

var a = 4/2;
var b=8/4; 
var c = 2; 
var d= 1*2;

are arr[2], arr[0+2], arr[1*2], arr[a], arr[b], arr[c], arr[d] same thing?

We often access array in a loop like this:

for (i=0; i<100; i++) {
  arr[i]=1;  // this is a[0],a[1] right?
  arr[i+0.0]=1;  // is this a[0] or a['0.0'] ?
}

If I write this:

for (i=0.1; i<100; i+=0.1) {
  arr[i*10]=1;  // what does it do?  a[1] = 1, a[1.0]=1 or a[1.00000] = 1 ?
}

what does the assignment in the loop do?

little offtopic: Why do you need to do so? What is practical value? — Damask
– Damask, Commented Oct 31, 2012 at 6:11

frosty · Accepted Answer · 2012-10-31 06:25:08Z

4

For starters, in JavaScript (ES5), there isn't such a thing as "Integer". JavaScript (ES5) only has Numbers.

Second, in JavaScript there is a lot of implicit type-casting going on. Here is an example:

if(1=='1') console.log('very truthy');

If you use a double-equals, it will cast the '1' to a Number and then it will compare the new value (1) to the 1 (which will be true, 1 == 1), and will then log the string 'very truthy'.

If you use triple-equals, the implicit type-casting won't happen.

if(1==='1') console.log("this won't get logged");

Using the triple-equals prevents the type casting from happening.

Next, when you add a value to a whole number index of an array, that index gets updated with the value you tell it to, AND THE LENGTH will get updated.

var a = [];
a[0] = 0
a[1] = 1; 
a[2.0] = 2;
//[undefined, 1, 2]

When you try to update an index that isn't a whole number (1.1), it will convert whatever that is to a string (1.1 becomes '1.1') and then it adds a new custom property to the array and set's the value on it. Custom properties of an array won't affect it's length.

var a = [];
a[1.1] = 1.1;
a.prop = "property";
//[], empty array
console.log(a.prop, a['1.1']); //"property",1.1

When you add a custom property to a JS array it mutates the object to then act like an object literal.

So in your case here, you end up with an array-ish/object-literal-ish mashup object. NOTE: If you add a custom property to a JS Number or String, they are NOT converted to Object Literals. This behavior that you are exploring is unique to JS arrays.

answered Oct 31, 2012 at 6:25

frosty

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2 Comments

Musa Over a year ago

The last paragraph made absolutely no sense to me.

frosty Over a year ago

@Musa Is there a piece of it specifically that you wanted to question about, or should we leave it at that?

generalhenry · Accepted Answer · 2012-10-31 06:08:53Z

3

JavaScript arrays aren't really arrays, they're JavaScript objects that have prototype methods that cause them to act like arrays. arr['one'] = 1 is valid JavaScript.

The way arr.length works is simply by looking at the array's keys, finding the largest number (JavaScript doesn't really do integers, just floats) and returning that number + 1.

try:

var arr = [];

arr.one = 1;
arr[8] = 1;
console.log(arr.length);

answered Oct 31, 2012 at 6:08

generalhenry

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8 Comments

Ord Over a year ago

Why do you say the largest number rather than largest integer? var arr = []; arr[6.5] = 10; arr.length; // gives 0

Damask Over a year ago

Seems like length attribute changed only when key can be clearly converted to integer.

user1787576 Over a year ago

But javascript:var a=[]; a[2]=1; a[8.0]=2; a[10.00001]=3; a[20.5]=4; alert(a.length); This alerts 9.

Ord Over a year ago

@user1787576 yes, because there is no difference between 8.0 and 8 in javascript.

user1600124 Over a year ago

Can't say I am sure.. but an array is also an object.. Under the javascript engine, perhaps it is implemented as an object with an array that uses integer key. So keys that can be turned into integer without losing information will be stored in the array.. where keys like 2.3, 1.001 are stored in the object portion that is not counted in the array.

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frosty · Accepted Answer · 2012-10-31 06:26:01Z

1

From playing around in Chrome's dev tools, I think your assertions aren't quite right. I found that:

arr[0] == arr[0.0] == ar[0.0000] == arr["0"]

BUT

arr[0] != arr["0.0"]
arr[0.0] != arr["0.0"]

So it appears that what the array is doing internally is calling .toString() on the index you give it, and using that as the actual index. Note that:

(0).toString() == "0"
(0.0000).toString() == "0"

edited Oct 31, 2012 at 6:26

frosty

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answered Oct 31, 2012 at 6:16

Ord

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