Array .slice() / concat()

var newArray = array.slice(startIndex, endIndex);

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

var members = ["item_a", "item_b", "item_c", "item_d", "item_e"];

// Extracts elements from index 1 up to (but not including) index 3.
var sliced = members.slice(1, 3);
console.log(sliced); // Outputs: "item_b,item_c"
console.log(members); // The original array is unchanged.

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

var copy = members.slice();
console.log(copy);

// Pattern 2: Combining arrays with concat
var items1 = ["item_a", "item_b"];
var items2 = ["item_c", "item_d"];
var merged = items1.concat(items2, ["item_e"]);
console.log(merged);
console.log(items1); // The original array is unchanged.

// Pattern 3: Practical use — pagination (splitting an array into pages)
var items3 = ["item_a", "item_b", "item_c", "item_d", "item_e", "item_f", "item_g", "item_h"];
var pageSize = 3;
var page1 = items3.slice(0, pageSize);
var page2 = items3.slice(pageSize, pageSize * 2);
console.log(page1); // Outputs: "item_a,item_b,item_c"
console.log(page2); // Outputs: "item_d,item_e,item_f"

var originals = ["item_a", "item_b", "item_c", "item_d"];
var backup = originals.slice(); // Save a copy first.
originals.splice(1, 2); // Modify the original array.
console.log(originals); // Outputs: "item_a,item_d"
console.log(backup); // The backup is intact.

[ 'item_b', 'item_c' ]
[ 'item_a', 'item_b', 'item_c', 'item_d', 'item_e' ]
[ 'item_d', 'item_e' ]
[ 'item_a', 'item_b', 'item_c', 'item_d', 'item_e' ]
[ 'item_a', 'item_b', 'item_c', 'item_d', 'item_e' ]
[ 'item_a', 'item_b' ]
[ 'item_a', 'item_b', 'item_c' ]
[ 'item_d', 'item_e', 'item_f' ]
[ 'item_a', 'item_d' ]
[ 'item_a', 'item_b', 'item_c', 'item_d' ]

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.