How to extract and merge multiple arrays from an array of object containing the array of interest?

 https://stackoverflow.com/questions/53149174/how-to-extract-and-merge-multiple-arrays-from-an-array-of-object-containing-the

You don't need flat function. You can use native array method.

data.reduce((acc: any, cur: any) => {
  return acc.concat(cur.subCategories.map((v: any) => v.label));
}, []);