Get index from coordinates, or vise versa with this simple trick.

array-matrix-get-functions.js

let matrix = new Array(width * height);

function coordinatesToIndex(x, y) {
    return x + (width * y);
}

function indexToCoordinates(index) {
    let x = index % width;
    let y = (index - x) / width;
    return { x: x, y: y };
}