dormd

flattenDeep([]) // => []

flattenDeep([[1]]) // => [1]

flattenDeep([1,[[[2],22,[222]]],3, () => {}, { a: 4 }, [undefined, [null]], null, 'hi']) 
// => [1, 2, 22, 222, 3, () => (), {a: 4}, undefined, null, null, 'hi']

dormd

range(0, 20, 5) // => [0, 5, 10, 15]

dormd

function range(start, end, step) {
  const results = [];
  
  if (end === undefined) {
    end = start;
    start = 0;
  }

  if (step === undefined) {
    step = (start < end) ? 1 : -1;

dormd

range(4) // => [0, 1, 2, 3]
 
range(-4) // => [0, -1, -2, -3]
 
range(1, 5) // => [1, 2, 3, 4]
 
range(0, 20, 5) // => [0, 5, 10, 15]
 
range(0, -4, -1) // => [0, -1, -2, -3]
 

dormd

const pizzaSlices = ['pepperoni', 'pepperoni', 'olives', 'olives', 'plain', 'plain', 'bacon', 'bacon'];
chunk(pizzaSlices, 2); // => [['pepperoni', 'pepperoni'], ['olives', 'olives'], ['plain', 'plain'], ['bacon', 'bacon']];
chunk(pizzaSlices, 3); // => [['pepperoni', 'pepperoni', 'olives'], ['olives', 'plain', 'plain'], ['bacon', 'bacon']];

dormd

function sumBy(items, iteratee) {
  const sumByFunc = getSumByFunc(iteratee);
  
  let sum = 0;
  items.forEach(item => {
    sum += sumByFunc(item);
  });
  
  return sum;
}

dormd

function sumBy(items, iteratee) {
  const sumByFunc = getSumByFunc(iteratee);
  
  return items.reduce((sum, item) => {
    return sum + sumByFunc(item);
  }, 0);
}

dormd

function sumBy(items, iteratee) {
  const sumByFunc = getSumByFunc(iteratee);
  
  // sum loop...
}

dormd

function sumBy(items, iteratee) {
  const sumByFunc = getSumByFunc(iteratee);
  
  let sum = 0;
  for(let i = 0; i < items.length; i++) {
    sum += sumByFunc(items[i]);
  }
  
  return sum;
}

dormd

function getSumByFunc(iteratee) {
  if (iteratee === undefined || iteratee === null) return item => item;
  if (typeof iteratee === 'string') return item => item[iteratee];
  return iteratee;
}