The “K Combinator".

tap_combinator_recipe.js

/**
 * tap takes a value and returns a function that
 * always returns the value, but if you pass it a
 * function, it executes the function for
 * side-effects.
 */

const tap = (value, fn) => {
  const curried = (fn) => (
      typeof(fn) === 'function' && fn(value),
      value
    );

  return fn === undefined
         ? curried
         : curried(fn);
};

/**
 * Example use case:
 *
 * The poor-man's debugger.
 */

// curried version
tap('espresso')((it) => {
  console.log(`Our drink is '${it}'`);
});

// uncurried version
tap('espresso', (it) => {
  console.log(`Our drink is '${it}'`);
});

// If we wish it to do nothing at all:

// curried version
tap('espresso')();

// uncurried version
tap('espresso', null)