Underscore.js: how to chain custom functions

Using Underscore.js, I can write the following which returns 42:

_([42, 43]).chain()
    .first()
    .value()

I have custom function, not part of Underscore.js called double():

function double(value) { return value * 2; };

I would like to be able to call this function in an Underscore chain, as if it was part of Underscore. I would like to write the following, which I'd like to return 84:

_([42, 43]).chain()
    .first()
    .double()
    .value()

This can't work since Underscore doesn't define double(). I could use tap() as in:

_([42, 43]).chain()
    .first()
    .tap(double)
    .value()

This is valid, but tap applies the function to its argument and returns the argument, not the result of the function. So it looks to me like I would need a sort of tap that returns the result of the function applied to its argument. Is there anything like this in Underscore.js? Am I missing something terribly obvious?