onSaveSpritesheet filenameChunks are a number?

[jednano]

This document says to write the following line of code:

var filenameChunks = spritesheet.groups.slice().push('png');

But then the value of filenameChunks would be a number.

Is this supposed to be something like this?

var filenameChunks = spritesheet.groups.concat('png');

My current onSaveSpritesheet callback looks like this:

onSaveSpritesheet: (opts, spritesheet) => {
    return join(
        opts.spritePath,
        spritesheet.groups.concat('png').join('.')
    );
}

Is that the right idea?