flatten.js

// guard against circular properties built in
function flatten(array) {
  const { isArray } = Array;
  if (!isArray(array)) {
    return [];
  }

  const visited = new WeakSet;
  const result = [];
  const stack = [array];

  do {
    const value = stack.pop();
    if (!isArray(value)) {
      result.push(value);
      continue;
    }

    if (visited.has(value)) {
      continue;
    }
    visited.add(value);

    for (const v of value) {
      stack.push(v);
    }
  } while (stack.length);

  return result.reverse();
}

// without guard against circular properties
function flatten(array) {
  const { isArray } = Array;
  if (!isArray(array)) {
    return [];
  }

  const result = [];
  const stack = [array];

  do {
    const value = stack.pop();
    if (!isArray(value)) {
      result.push(value);
      continue;
    }

    for (const v of value) {
      stack.push(v);
    }
  } while (stack.length);

  return result.reverse();
}