user-supplied-functional-options.md

User-Supplied Functional Options

User-supplied functional options are functions that can modify your function's return value before your function returns.

• They are identity functions.
• They are passed into your function as a rest parameter.

For example:

export function makeNominalType<IN, OUT>(
    contract: DataGuarantee<IN>,
    input: IN,
    { onError = THROW_THE_ERROR }: OnErrorOptions = {},
    ...fnOpts: FunctionalOption<IN>[]
) {
    // apply the contract
    contract(input, { onError });

    // apply any functional options we have
    let retval = input;
    fnOpts.forEach((fnOpt) => { retval = fnOpt(retval, { onError }); });

    // all done
    return (retval as unknown) as OUT;
}

// an example of how we make use of functional options
const makeContentType = (input: string, options: OnErrorOptions)
    => makeNominalType<string, ContentType>(
        mustBeContentType,
        input,
        options,
        // these are functional options
        String.prototype.trim,
        String.prototype.toLowerCase
    );

This is a concept that we've adopted from the Golang community. See Functional options for friendly APIs to learn more.