async function saveItem(item) {
        const contents = await readFile()
        const idx = contents.findIndex(x => x.id == item.id)
        if(idx > -1) {
            contents.splice(idx, 1)
        }
        contents.push(item)
        return await writeFile(contents)
    }
@param {{id: number|string}} item 
@returns {Promise<void>}