Flatten an array of arbitrarily nested elements.

flatten.js

// Flattens an array of arbitrarily nested elements.
// E.g. [[1,2,[3]],4] -> [1,2,3,4]
//
// - Doesn't work with negative array indices!
// - ES6 TCO requires recursion to stay in tail-call position.
function flatten(xs) {2
  if(!Array.isArray(xs)) {
    return xs;
  }
  
  function flatConcat(xs, x) {
    return xs.concat(flatten(x));
  }
  
  return xs.reduce(flatConcat, []);
}

tests.js

const assert = require('assert');

assert.deepEqual(
  flatten([1,2,3,4]),
  [1,2,3,4]
);

assert.deepEqual(
  flatten([[1,2,[3]],4]),
  [1,2,3,4]
);

assert.deepEqual(
  flatten([[[[1]]]]),
  [1]
);

assert.equal(
  flatten(1),
  1
);