Polyfill Array.flat

A polyfill is a piece of code that provides functionality that's not natively supported by a web browser.

If you're working with a browser that doesn't support the flat() method, you can use a polyfill to provide the same functionality. Here's an example of how you can create a polyfill for the flat() method:

if (!Array.prototype.flat) {
  Array.prototype.flat = function(depth) {
    var flattened = [];

    function flatten(arr, currentDepth) {
      for (var i = 0; i < arr.length; i++) {
        if (Array.isArray(arr[i]) && currentDepth < depth) {
          flatten(arr[i], currentDepth + 1);
        } else {
          flattened.push(arr[i]);
        }
      }
    }

    flatten(this, 0);

    return flattened;
  };
}

The implementation of the flat() method is done recursively. The flatten() function takes an array arr and a current depth currentDepth. If the arr is an array and the currentDepth is less than depth, then the flatten() function is called recursively with the nested array and the currentDepth incremented by 1. Otherwise, the elements of the array are pushed onto the flattened array.

Usage

var nestedArray = [
  [1, 2, 3],
  [4, 5, [6, 7, [8, 9, [10]]]],
  [7, 8, 9],
];

nestedArray.customFlat(2) 

// [ 1, 2, 3, 4, 5, 6, 7, [ 8, 9, [ 10 ] ], 7, 8, 9 ]