Flatten an arbitrarily nested array

flatten.js

function flattenArr(nestedArr, retVal = []) {
  nestedArr.forEach(val => {
    // this should be the only check since no object are expected
    const isArr = (typeof val === 'object');
    if(isArr) {
      flattenArr(val, retVal);
    } else {
      retVal.push(val);
    }
  });
  return retVal;
}