Ambiguous array assignments in JavaScript

Question

After watching this question, I came to know

let a = [1, 2] [3, 4] = [5, 6];

console.log(a);

is valid JavaScript code but, I'm not able to understand, how it is working. Can anyone explain working flow of above code.

Answer 1

After executing a lot of variations of

let a = [1, 2] [3, 4] = [5, 6];

I came to know

[1, 2] [3, 4] is equivalent to [1, 2][4] where [1, 2] represents an array while [4] represents 5th index of [1, 2] array but could not find any official documentation.

Update I was wrong about documentation as mentioned in comments, [] in [3, 4] is Property accessor while 3, 4 are two different code statements separated by Comma operator. In comma operator, every statement separated by , is executed from left to right and result of last statement is returned.

Above code now simplifies to

let a = [1, 2][4] = [5, 6];

which is comparatively easy to understand. If we simplify it little bit more it becomes

let arr = [1, 2];
let a = arr[4] = [5, 6];

console.log(a);
console.log(arr);

Now it is very easy to explain it arr[4] = [5, 6] assigns [5, 6] to 5th index of arr and a = arr[4] assigns 5th index of arr to a which is now [5, 6] so [5, 6] is assigned to a.

Now current value of a is [5, 6] and current value of arr is [ 1, 2, undefined, undefined, [ 5, 6 ] ]

Lets rewrite above code again

let arr = [1, 2];
let a = arr[3, 4] = [5, 6];

console.log(a);
console.log(arr);