Array flattener

flatten_array_with_tests.js

// You can test it in the browser here: https://codepen.io/svschannak/pen/KoLZvm?editors=0011
// try to avoid using const and arrow functions to declare an function, it will help you while debugging
function flatten_array(originList){
    /**
   * Flattens an input list with multi levels.
   * WORKS only with ES6
   * Description. (use period)
   * @param {Array[List]} originList  Array List to be flattened.
   * 
   * @return {Array[List]} New flattened array list.
   */
    let returnList = [];
    let currentItem;
    
    // walk through all array elements
    while (currentItem = originList.shift()) {
      // check if the current item is an array 
      if (Array.isArray(currentItem)) {
        // try to flatten the next list item
        const flattenedList = flatten_array(currentItem);
        // merge flattened list and current list
        returnList = returnList.concat(flattenedList);
        
      } else {
        // if current item is not a list, push the item to the return list
        returnList.push(currentItem);
      }
    }

    return returnList;
}

const TEST_CASES = [
  { origin: [[1,2,[3]],4], result: [1,2,3,4], shouldNotFail: true},
  { origin: [[1,2,[[3]]],4], result: [1,2,3,4], shouldNotFail: true },
  // also check false positives
  { origin: [[1,2,[[3]]],5], result: [1,2,3,4], shouldNotFail: false }
]

function checkIfArraysEqual(arr1, arr2) {
    if(arr1.length !== arr2.length){
        return false;
    }
    for(var i = arr1.length; i--;) {
        if(arr1[i] !== arr2[i])
            return false;
    }

    return true;
}

TEST_CASES.forEach((testCase, idx) => {
  const flattened_array = flatten_array(testCase['origin']);
  const check = checkIfArraysEqual(flattened_array, testCase['result']);
  
  if(check === testCase['shouldNotFail']){
      console.log(`TEST for testcase no. ${idx + 1} SUCCESSFUL`);
  }else{
      console.log(`TEST for testcase no. ${idx + 1} FAILED`);
  }
  
})