flatten.js

// http://jsbin.com/lirise/edit?js,output

function flatten (arr){

  if (!Array.isArray(arr)){
    if (arguments.length === 2) {
      return arr;
    } else {
      throw new TypeError('flatten accepts only arrays');
    }
  }
  
  return arr.reduce(function(a, b) {
    return a.concat(flatten(b, 'internal call'));
  }, []);

}


QUnit.test( "Flatten Array", function( assert ) {
  var example0 = ['Hire',[],'me'];
  var example1 = [[1,2,[3,[[1]]],4]];
  
  assert.throws( function() {flatten();});
  assert.throws( function() {flatten({});});
  
  assert.deepEqual(flatten([]), []);
  assert.deepEqual(flatten(['0']), ['0']);
  assert.deepEqual(flatten([0,1,3]), [0,1,3]);
  assert.deepEqual(flatten([[[1]]]), [1]);
  
  assert.deepEqual(flatten(example0), ['Hire', 'me']);
  assert.deepEqual(flatten(example1), [1,2,3,1,4]);
 
});

http://jsbin.com/lirise/edit?js,output