Array.prototype.concat()

const array1 = ["a", "b", "c"];
const array2 = ["d", "e", "f"];
const array3 = array1.concat(array2);

console.log(array3);
// Expected output: Array ["a", "b", "c", "d", "e", "f"]

concat()
concat(value0)
concat(value0, value1)
concat(value0, value1, /* , */ valueN)

const letters = ["a", "b", "c"];
const numbers = [1, 2, 3];

const alphaNumeric = letters.concat(numbers);
console.log(alphaNumeric);
//  ['a', 'b', 'c', 1, 2, 3]

const num1 = [1, 2, 3];
const num2 = [4, 5, 6];
const num3 = [7, 8, 9];

const numbers = num1.concat(num2, num3);

console.log(numbers);
//  [1, 2, 3, 4, 5, 6, 7, 8, 9]

const letters = ["a", "b", "c"];

const alphaNumeric = letters.concat(1, [2, 3]);

console.log(alphaNumeric);
//  ['a', 'b', 'c', 1, 2, 3]

const num1 = [[1]];
const num2 = [2, [3]];

const numbers = num1.concat(num2);

console.log(numbers);
//  [[1], 2, [3]]

// num1    .
num1[0].push(4);

console.log(numbers);
//  [[1, 4], 2, [3]]

const obj1 = { 0: 1, 1: 2, 2: 3, length: 3 };
const obj2 = { 0: 1, 1: 2, 2: 3, length: 3, [Symbol.isConcatSpreadable]: true };
console.log([0].concat(obj1, obj2));
// [ 0, { '0': 1, '1': 2, '2': 3, length: 3 }, 1, 2, 3 ]

console.log([1, , 3].concat([4, 5])); // [1, empty, 3, 4, 5]
console.log([1, 2].concat([3, , 5])); // [1, 2, 3, empty, 5]

console.log(Array.prototype.concat.call({}, 1, 2, 3)); // [{}, 1, 2, 3]
console.log(Array.prototype.concat.call(1, 2, 3)); // [ [Number: 1], 2, 3 ]
const arrayLike = {
  [Symbol.isConcatSpreadable]: true,
  length: 2,
  0: 1,
  1: 2,
  2: 99, // length 2 concat() 
};
console.log(Array.prototype.concat.call(arrayLike, 3, 4)); // [1, 2, 3, 4]