JS Machine Coding Questions — Implementations You Must Know

Flatten — object + array paths (5.3)

Same as 5.2, but nested arrays become [index] segments in the key string (e.g. w[0].e.r).
Mix of dots (object keys) and brackets (array indices) in one string.
Three separate questions: 5.1 array flattening, 5.2 object-only dot paths, 5.3 object + array path keys.

function flattenObject(obj, prefix = "") {
  const out = {};
  if (obj === null || typeof obj !== "object") {
    if (prefix !== "") out[prefix] = obj;
    return out;
  }
  if (Array.isArray(obj)) {
    if (obj.length === 0) {
      if (prefix !== "") out[prefix] = [];
      return out;
    }
    obj.forEach((item, i) => {
      const path = prefix ? `${prefix}[${i}]` : `[${i}]`;
      Object.assign(out, flattenObject(item, path));
    });
    return out;
  }
  for (const k of Object.keys(obj)) {
    const path = prefix ? `${prefix}.${k}` : k;
    const v = obj[k];
    if (v !== null && typeof v === "object") {
      Object.assign(out, flattenObject(v, path));
    } else {
      out[path] = v;
    }
  }
  return out;
}

flattenObject({ w: [{ e: { r: 8 } }], x: { y: 2 } });
// { "w[0].e.r": 8, "x.y": 2 }