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

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);