Arbitrary precision Maidenhead grid to lat/lon conversion in JavaScript

gistfile1.txt

// Convert a Maidenhead grid reference of arbitrary previcision to lat/long.
function latLonForGrid(grid) {
    // Make sure we are in upper case so our maths works. Case is arbitrary for Maidenhead references
    grid = grid.toUpperCase();

    // Return null if our Maidenhead string is invalid or too short
    let len = grid.length;
    if (len <= 0 || (len % 2) !== 0) {
        return null;
    }

    let lat = 0.0; // aggregated latitude
    let lon = 0.0; // aggregated longitude
    let latCellSize = 10; // Size in degrees latitude of the current cell. Starts at 20 and gets smaller as the calculation progresses
    let lonCellSize = 20; // Size in degrees longitude of the current cell. Starts at 20 and gets smaller as the calculation progresses
    let latCellNo; // grid latitude cell number this time
    let lonCellNo; // grid longitude cell number this time

    // Iterate through blocks (two-character sections)
    for (let block = 0; block * 2 < len; block += 1) {
        if (block % 2 === 0) {
            // Letters in this block
            lonCellNo = grid.charCodeAt(block * 2) - 'A'.charCodeAt(0);
            latCellNo = grid.charCodeAt(block * 2 + 1) - 'A'.charCodeAt(0);
        } else {
            // Numbers in this block
            lonCellNo = parseInt(grid.charAt(block * 2));
            latCellNo = parseInt(grid.charAt(block * 2+1));
        }

        // Aggregate the angles
        lat += latCellNo * latCellSize;
        lon += lonCellNo * lonCellSize;

        // Reduce the cell size for the next block, unless we are on the last cell. If we are on the last cell, instead
        // move the position into the middle of the cell rather than its south-west corner.
        if (block * 2 < len - 2) {
            // Still have more work to do, so reduce the cell size
            if (block % 2 === 0) {
                // Just dealt with letters, next block will be numbers so cells will be 1/10 the current size
                latCellSize = latCellSize / 10.0;
                lonCellSize = lonCellSize / 10.0;
            } else {
                // Just dealt with numbers, next block will be letters so cells will be 1/24 the current size
                latCellSize = latCellSize / 24.0;
                lonCellSize = lonCellSize / 24.0;
            }
        } else {
            // This is the last block, so move the marker to the middle.
            lat += latCellSize / 2;
            lon += lonCellSize / 2;
        }
    }

    // Offset back to (-180, -90) where the grid starts
    lon -= 180.0;
    lat -= 90.0;

    return [lat, lon];
}