How to use the @ethersproject/bignumber.formatFixed function in @ethersproject/bignumber

export function formatUnits(value, unitName) {
    if (typeof (unitName) === "string") {
        const index = names.indexOf(unitName);
        if (index !== -1) {
            unitName = 3 * index;
        }
    }
    return formatFixed(value, (unitName != null) ? unitName : 18);
}
export function parseUnits(value, unitName) {

export function formatUnits(value: BigNumberish, unitName?: string | BigNumberish): string {
    if (typeof(unitName) === "string") {
        const index = names.indexOf(unitName);
        if (index !== -1) { unitName = 3 * index; }
    }
    return formatFixed(value, (unitName != null) ? unitName: 18);
}