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?

Accepted Answer

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);
```

How does JavaScript determine if an array index is integer?

Solution

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