Cap function n params. Useful to cap iteratee for example.

cap.js

const capTo = n => fn => (...args) => fn(...args.slice(0, n));
const capRightTo = n => fn => (...args) => fn(...args.slice(-n));
const cap = capTo(0);

const add = (a, b) => a + b;
const sum = (...args) => args.reduce(add, 0);

// Simple Example

sum(1, 2, 3, 4);                  // --> 10

cap(sum)();                       // --> 0
capTo(1)(sum)(1, 2, 3, 4);        // --> 1
capTo(2)(sum)(1, 2, 3, 4);        // --> 3
capTo(3)(sum)(1, 2, 3, 4);        // --> 6
capTo(4)(sum)(1, 2, 3, 4);        // --> 10

capRightTo(1)(sum)(1, 2, 3, 4);   // --> 4
capRightTo(2)(sum)(1, 2, 3, 4);   // --> 7
capRightTo(3)(sum)(1, 2, 3, 4);   // --> 9
capRightTo(4)(sum)(1, 2, 3, 4);   // --> 10