pickCorrectOffset.js

const isntEmpty = complement(isEmpty);
const isntNil = complement(isNil);
const isntNaN = complement(isNaN);

const pageLens = lensProp('page');

const numberValidations = [
  isntNaN,
  isntNil,
  isntEmpty,
  is(Number),
];

const numberValidationsGT0 = concat(numberValidations, [lt(0)]);

const integerValidations = [Number.isInteger];

const integerValidationsGT0 = concat(integerValidations, [lt(0)]);

const calcPageOffset = compose(
  apply(multiply),
  values,
  over(pageLens, dec),  
  pick(['page', 'limit']),
);

const validNumberGT0At = (keyName) => compose(
  allPass(numberValidationsGT0),
  prop(keyName),
);

const validIntegerGT0At = (keyName) => compose(
  allPass(integerValidationsGT0),
  prop(keyName),
);

const hasValidPageOffsetData = allPass([
  validNumberGT0At('page'),
  validNumberGT0At('limit')
]);

const hasValidOffset = validIntegerGT0At('offset');

const getOffset = cond([
  [hasValidPageOffsetData, calcPageOffset],
  [hasValidOffset, prop('offset')],
  [T, always(0)]
]);


const data = {
  page: 25,
  offset: 5,
  limit: 10,
};

getOffset(data);