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?

Answer 1

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.

Answer 2

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);