Array .slice() / concat()

// Extracts part of an array and returns it as a new array.
var newArray = array.slice(startIndex, endIndex);

// Combines multiple arrays or values into a new array.
var newArray = array.concat(array2, array3, ...);

var fruits = ["apple", "orange", "grape", "banana", "peach"];

// Extracts elements from index 1 up to (but not including) index 3.
var sliced = fruits.slice(1, 3);
console.log(sliced); // Outputs: "orange,grape"
console.log(fruits); // Outputs: "apple,orange,grape,banana,peach" — the original array is unchanged.

// A negative value counts from the end of the array.
var last2 = fruits.slice(-2);
console.log(last2); // Outputs: "banana,peach"

// Omitting arguments copies the entire array.
var copy = fruits.slice();
console.log(copy); // Outputs: "apple,orange,grape,banana,peach"

// Combines arrays.
var arr1 = [1, 2];
var arr2 = [3, 4];
var merged = arr1.concat(arr2, [5, 6]);
console.log(merged); // Outputs: "1,2,3,4,5,6"

// You can also append individual values directly.
var added = arr1.concat(3, 4);
console.log(added); // Outputs: "1,2,3,4"

Both array.slice() and array.concat() are non-destructive methods — they return a new array without modifying the original. They are useful when you want to work with array data safely.

array.slice() takes a start index as the first argument and an end index as the second. Note that the element at the end index is not included in the result. A negative value counts from the end of the array, so slice(-2) returns the last two elements. When called with no arguments, it creates a shallow copy of the array, making it a handy way to copy an array without affecting the original.

The similarly named array.splice() is a destructive method that modifies the original array directly. Use array.slice() when you only need to read elements, and array.splice() when you need to add, remove, or replace elements.