This statement gets the index of the last index of the array plus one. It is used to append items to an array. Equivalent to array.push().

var fruits = ["apple", "orange", "banana", "pear"];
// appends grapes to the array
fruits[fruits.length] = "grapes";
console.log(fruits);

For further reading, check out W3School's page on array methods.

arr.length is the length of the array.

arr[arr.length] is accessing by index the array length (out of bounds as length starts at 1, not index 0).

example:

const test = [1,2,3]
test[test.length]
undefined
test[test.length-1]
3

arr.length means the length of the array that spans from index 0 to arr.length - 1.

For instance if you got arr = ["a", "b", "c"], arr.length is 3.

arr[0] is "a", arr[1] is "b" and arr[2] is "c".

arr[3] does not exist, yet. but if you do arr[arr.length] = "d", you obtain ["a", "b", "c", "d"].

This is an odd construction and usually, one should simply write arr.push("d").

const arr = ["a", "b", "c"];
console.log(arr.length, arr[0], arr[1], arr[2], arr[3]);
arr[arr.length] = "d";
console.log(arr);
arr.push("e");
console.log(arr);