Array .splice()

// Removes elements starting at the specified index.
var removed = array.splice(startIndex, deleteCount);

// Inserts elements at the specified index without removing any.
array.splice(startIndex, 0, item1, item2, ...);

// Replaces elements at the specified index.
array.splice(startIndex, deleteCount, item1, item2, ...);

sample_splice.js

// Pattern 1: Removing elements
var members = ["Kiryu Kazuma", "Majima Goro", "Akiyama Shun", "Nishikiyama Akira", "Saejima Taiga"];

// Removes 2 elements starting at index 1.
var removed = members.splice(1, 2);
console.log(removed); // Outputs "Majima Goro,Akiyama Shun".
console.log(members); // Outputs "Kiryu Kazuma,Nishikiyama Akira,Saejima Taiga".

// Pattern 2: Inserting elements
// Inserts elements at index 1 without removing any.
members.splice(1, 0, "Majima Goro", "Akiyama Shun");
console.log(members); // Outputs "Kiryu Kazuma,Majima Goro,Akiyama Shun,Nishikiyama Akira,Saejima Taiga".

// Pattern 3: Replacing elements
// Replaces the element at index 2.
members.splice(2, 1, "Kiryu Kazuma");
console.log(members); // Outputs "Kiryu Kazuma,Majima Goro,Kiryu Kazuma,Nishikiyama Akira,Saejima Taiga".

// Pattern 4: Using a negative index to count from the end
var roster = ["Kiryu Kazuma", "Majima Goro", "Akiyama Shun", "Nishikiyama Akira", "Saejima Taiga"];
// Removes the second-to-last element (Nishikiyama Akira).
var popped = roster.splice(-2, 1);
console.log(popped); // Outputs "Nishikiyama Akira".
console.log(roster); // Outputs "Kiryu Kazuma,Majima Goro,Akiyama Shun,Saejima Taiga".

// Pattern 5: Practical use — find and replace a specific element
var enemies = ["Kurita", "Shimano", "Kiryu Kazuma", "Omi Alliance"];
// Find the index of "Kiryu Kazuma" and replace it.
var idx = enemies.indexOf("Kiryu Kazuma");
if (idx !== -1) {
	enemies.splice(idx, 1, "Nishikiyama Akira");
}
console.log(enemies); // Outputs "Kurita,Shimano,Nishikiyama Akira,Omi Alliance".

[ 'Majima Goro', 'Akiyama Shun' ]
[ 'Kiryu Kazuma', 'Nishikiyama Akira', 'Saejima Taiga' ]
[ 'Kiryu Kazuma', 'Majima Goro', 'Akiyama Shun', 'Nishikiyama Akira', 'Saejima Taiga' ]
[ 'Kiryu Kazuma', 'Majima Goro', 'Kiryu Kazuma', 'Nishikiyama Akira', 'Saejima Taiga' ]
[ 'Nishikiyama Akira' ]
[ 'Kiryu Kazuma', 'Majima Goro', 'Akiyama Shun', 'Saejima Taiga' ]
[ 'Kurita', 'Shimano', 'Nishikiyama Akira', 'Omi Alliance' ]

array.splice() is a versatile method that can remove, insert, and replace elements at any position in an array in a single call. It is a mutating method that modifies the original array in place, and its return value is an array of the removed elements. If no elements were removed, an empty array is returned.

If you specify a negative value for the start index, it is treated as an offset from the end of the array. For example, -1 refers to the last element, and -2 refers to the second-to-last element. If you omit the second argument, all elements from the start index to the end are removed, which is convenient when you want to truncate the end of an array.

If you want to extract a portion of an array without modifying the original, use array.slice() instead. The names look similar, but there is an important difference: array.splice() modifies the original array, while array.slice() does not.