Unary takes any function and turns it into a function taking exactly one argument.

unary_decorator_recipe.js

const unary = (fn) =>
  fn.length === 1
    ? fn
    : function (something) {
        return fn.call(this, something);
      };

/**
 * Example use case
 * 
 * JavaScript’s map calls each function with
 * three arguments: The element, the index of
 * the element in the array, and the array
 * itself.
 */

// If you pass in a function taking only one argument,
// it simply ignores the additional arguments
['1', '2', '3'].map(parseFloat);
  //=> [1, 2, 3]

// But some functions have optional second or even third
// arguments.
['1', '2', '3'].map(parseInt);
  //=> [1, NaN, NaN]

// This doesn’t work because parseInt is defined as
// parseInt(string[, radix]). It takes an optional radix
// argument. And when you call parseInt with map, the index
// is interpreted as a radix.

// To fix it use the unary decorator.
['1', '2', '3'].map(unary(parseInt));
  //=> [1, 2, 3]