(JS) array flat

1. If there is only one layer of nested arrays

Array.prototype.concat.apply([], arr)
 or 
[].concat(arr)
 or 
Array.prototype.concat.call([], arr[0],arr[1]...)

2. If there are multiple nesting

function flat(arr) {
    
		// Determine whether there is a multi-layer array 
      const isDeep = arr.some(item => {
    
        return item instanceof Array;
      });
      // If there is no multi-level array , Then return to arr
      if (!isDeep) {
    
        return arr;
      }
      const res = Array.prototype.concat.apply([], arr);
      return flat(res)
    }