E0450: misleading use of ',' operator in index

Using comma operator within an indexing operation is misleading, as it can be confused with a new array definition, or with access to multiple elements of the container.

For example:

let matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9],
];
let topLeft = matrix[0, 0];

Here, the value of topLeft will be equal to matrix[0] = [1, 2, 3]. If the intention is to find the value at the top-left corner of matrix, the correct syntax would be:

let matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9],
];
let topLeft = matrix[0][0]; // topLeft = 1

Documentation for other errors