Array.prototype.reduceRight()

const array1 = [
  [0, 1],
  [2, 3],
  [4, 5],
];

const result = array1.reduceRight((accumulator, currentValue) =>
  accumulator.concat(currentValue),
);

console.log(result);
// Expected output: Array [4, 5, 2, 3, 0, 1]

    arr.reduceRight(callback[, initialValue])

array.reduceRight(function (previousValue, currentValue, index, array) {
  // ...
});

[0, 1, 2, 3, 4].reduceRight(
  function (previousValue, currentValue, index, array) {
    return previousValue + currentValue;
  },
);

[0, 1, 2, 3, 4].reduceRight(function (
  previousValue,
  currentValue,
  index,
  array,
) {
  return previousValue + currentValue;
}, 10);

var sum = [0, 1, 2, 3].reduceRight(function (a, b) {
  return a + b;
});
// sum is 6

var flattened = [
  [0, 1],
  [2, 3],
  [4, 5],
].reduceRight(function (a, b) {
  return a.concat(b);
}, []);
// flattened is [4, 5, 2, 3, 0, 1]

var a = ["1", "2", "3", "4", "5"];
var left = a.reduce(function (prev, cur) {
  return prev + cur;
});
var right = a.reduceRight(function (prev, cur) {
  return prev + cur;
});

console.log(left); // "12345"
console.log(right); // "54321"

// ECMA-262, 5 , 15.4.4.22  
//  : http://es5.github.io/#x15.4.4.22
if ("function" !== typeof Array.prototype.reduceRight) {
  Array.prototype.reduceRight = function (callback /*, initialValue*/) {
    "use strict";
    if (null === this || "undefined" === typeof this) {
      throw new TypeError("Array.prototype.reduce called on null or undefined");
    }
    if ("function" !== typeof callback) {
      throw new TypeError(callback + " is not a function");
    }
    var t = Object(this),
      len = t.length >>> 0,
      k = len - 1,
      value;
    if (arguments.length >= 2) {
      value = arguments[1];
    } else {
      while (k >= 0 && !(k in t)) {
        k--;
      }
      if (k < 0) {
        throw new TypeError("Reduce of empty array with no initial value");
      }
      value = t[k--];
    }
    for (; k >= 0; k--) {
      if (k in t) {
        value = callback(value, t[k], k, t);
      }
    }
    return value;
  };
}