Practice Questions

Questions related to flatten array

"Flattening" an array means converting nested arrays into a single flat array containing all elements without nesting.

What does it mean to "flatten" an array?

It's the process of converting a multi-dimensional array ([1, [2, [3]]]) into a one-dimensional one ([1, 2, 3]).

2. How to implement flatten without built-in methods?

With recursion:

function flatten(arr) {
  return arr.reduce(
    (acc, val) => acc.concat(Array.isArray(val) ? flatten(val) : val),
    []
  );
}

Here reduce combines elements, recursively flattening nested arrays.

3. How to use the built-in `flat()` method?

[1, [2, [3, [4]]]].flat(2); // [1, 2, 3, [4]]
[1, [2, [3, [4]]]].flat(Infinity); // completely flattens

The depth parameter determines the flattening depth.

4. Can you flatten without recursion?

Yes, using a stack (iterative way):

function flattenIterative(arr) {
  const stack = [...arr];
  const res = [];
  while (stack.length) {
    const next = stack.pop();
    Array.isArray(next) ? stack.push(...next) : res.push(next);
  }
  return res.reverse();
}

5. What are typical mistakes?

Skipping the Infinity parameter in flat() if the depth is unknown.
Using concat without considering nested levels.
Mutating the original array during iteration.

https://devtools.tech/questions/s/how-to-implement-event-emitter-in-javascript-or-facebook-interview-question---qid---J4fNqXImp6QIdGMc7SPF