Flatten a Nested Array of Arbitrary Depth

[sudoUgando]

Flatten a Nested Array of Arbitrary Depth
Do it without using .flat(), and handle sparse arrays.

[EimanTahir071]

Flatten a Nested Array of Arbitrary Depth
Do it without using .flat(), and handle sparse arrays.

a utility to flatten nested arrays of arbitrary depth without using .flat(), while also preserving sparseness (holes stay holes).

js
/**

• Flatten a nested array of arbitrary depth (no .flat()).
• Preserves sparse arrays: array holes remain holes in the result.
• @PARAM {any[]} input
• @returns {any[]} flattened array
  */
  function flattenSparse(input) {
  if (!Array.isArray(input)) throw new TypeError("Expected an array");

const result = [];
const stack = [{ arr: input, idx: 0 }];

while (stack.length) {
const frame = stack[stack.length - 1];
const { arr } = frame;

if (frame.idx >= arr.length) {
  stack.pop();
  continue;
}

const i = frame.idx++;
if (!(i in arr)) {
  // Preserve hole
  result.push(undefined); // placeholder
  // We'll fix it to a real hole (delete the index) after.
  continue;
}

const value = arr[i];

if (Array.isArray(value)) {
  // Descend into nested array
  stack.push({ arr: value, idx: 0 });
} else {
  result.push(value);
}

}

// Convert placeholders that came from holes into actual holes.
// We detect them by comparing against the original traversal rules:
// Holes were pushed as undefined; but real undefined values should remain.
// To distinguish, we need to track which pushes corresponded to holes.
//
// So: we redo with a version that records hole positions.
return flattenSparseWithHoleTracking(input);
}

function flattenSparseWithHoleTracking(input) {
const result = [];
const holePositions = new Set();

const stack = [{ arr: input, idx: 0 }];

while (stack.length) {
const frame = stack[stack.length - 1];
const { arr } = frame;

if (frame.idx >= arr.length) {
  stack.pop();
  continue;
}

const i = frame.idx++;
if (!(i in arr)) {
  // Preserve hole: record position in result as a hole
  holePositions.add(result.length);
  result.push(undefined); // placeholder; later converted to hole
  continue;
}

const value = arr[i];
if (Array.isArray(value)) {
  stack.push({ arr: value, idx: 0 });
} else {
  result.push(value);
}

}

// Turn placeholders into actual holes.
for (const pos of holePositions) {
delete result[pos];
}

return result;
}