[JavaScript] Create a function to flatten a multi dimensional array with numbers, and remove any null/undefined value if present

flattenArray.js

function flattenArray(arr, res) {
  let result = [];
  
  for(let index = 0; index < arr.length; index++) {
    const currentValue = arr[index];
    if(Array.isArray(currentValue)) {
      result = [...result, ...(flattenArray(currentValue))];
      // we can also use concat method to achieve the same result
      // result = result.concat(flattenArray(currentValue));
    } else {
      if(currentValue != null) {
        result.push(currentValue);
      }
    }
  }
  
  return result;
  
}

const testArray = [1, 2, null, [4, undefined, [11, 10]], 6, [7, null, 0], null, 9];

const result = flattenArray(testArray);

console.log(result);  // [1, 2, 4, 11, 10, 6, 7, 0, 9]


/** we also have flat method on Array prototype which flattens the Array, and we can provide a parameter to decide the depth of array till which it should be flattened.
Here, we will again need to write logic to remove the null/undefined values */

console.log(testArray.flat()) // [1, 2, null, 4, undefined, [11, 10], 6, 7, null, 0, null, 9]

/** if we do not know the depth of Array, and want it flattened then we can pass `Infinity` as argument */

console.log(testArray.flat(Infinity)) // [1, 2, null, 4, undefined, 11, 10, 6, 7, null, 0, null, 9]


// we can use filter to eliminate all null/undefined items from the flattened array
const result = testArray.flat(Infinity).filter(val => val != null);
console.log(result);  // [1, 2, 4, 11, 10, 6, 7, 0, 9]



/** for more information on .flat method, visit below link:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat */