How to Use JavaScript Array Splice

This article was last updated on September 05, 2024, to add sections on Splice vs. Slice, Edge Cases with Splice, Performance Considerations, Splice in Multidimensional Arrays, and Using Splice with Destructuring.

Introduction​

Array.prototype.splice() is a JavaScript Array method that is used to remove, replace or insert items starting at a chosen index. It is also an effective tool for adding items to the end of an array.

Array.prototype.splice() is a sparsely used but powerful JavaScript method for manipulating arrays in place. This means, we are mutating the actual array, not a copy of it. JavaScript splice() is useful for wiping all items after n number of initial items. It is handy for removing a number of intermediary items starting at an indicated index. It is especially needed for inserting items when we want to replace items or overload an array besides replacement. JavaScript splice() offers an alternative to Array.prototype.push() for growing the tail of the array as well.

Unlike several other Array methods, JavaScript slice() does not have a String equivalent.

In this post, we play around to illustrate various use cases of JavaScript splice() with trivial examples. We start with the method signature for Array.prototype.splice(), its possible parameters (startIndex, deleteCount, items) and what they represent. We then go ahead and see examples of using combinations of passed arguments to manipulate an array. In particular, we observe how to wipe out items from a certain index onward. We also go through examples of replacing existing items with new ones. We explore how to insert items in the case of overloaded replacement and injection. We also figure out splice() way of adding items to the tail of the array.

Towards the end, we touch base on the nuances of the values passed as startIndex and learn how JavaScript splice() behaves when startIndex argument is negative or greater than the array length.

Steps we'll cover in this post:

• JavaScript Splice
  • JS Array.prototype.splice() Method Signature
  • JS splice() Result / Return Value
  • JS Array.prototype.splice() without Arguments
  • Wiping All After n Items - JavaScript splice() with Only startIndex
  • Deleting Intermediary Items with JS Array.prototype.splice()
  • Replacing Items with JS Array.prototype.splice()
  • Items Injection with Array.prototype.splice()
  • Tailgrowing with Array.prototype.splice()
  • Inject Items Before Last n Items - JavaScript slice() with Negative startIndex
• Nuances of JS Array.prototype.splice()
• Bonus: Splice vs. Slice in JavaScript
• Splice on Multidimensional Arrays
• Using Splice with Destructuring in JavaScript

JavaScript Splice​

When called by an array, JavaScript splice() takes three possible arguments: startIndex, deleteCount and items list. Of these three types of arguments, startIndex indicates a zero-based index at which mutation kicks off. deleteCount denotes the number of items to be deleted and items is a possibly infinite list of items to be inserted.

JS Array.prototype.splice() Method Signature​

Possible call signatures for JavaScript splice() are:

splice()
splice(startIndex)
splice(startIndex, deleteCount, item1)
splice(startIndex, deleteCount, item1, item2, /*...*/, itemN)

None of the arguments for Array.prototype.splice() are required. However, when we want to mutate the caller array, we have to pass the startIndex as a minimum. Passing deleteCount depends on whether we want to delete any item/s or not. And passing the items list depends on whether/what we want to insert or add to the array. It is also possible to pass all arguments at the same time. In the coming sections, we consider most of the cases.

JS splice() Result / Return Value​

As we'll start seeing next, splice() mutates the original array and returns an array containing removed items.

JS Array.prototype.splice() without Arguments​

Although, JavaScript splice() attempts to mutate the caller array, the returned value is an array of removed items. We can get a hint of the return value when an array calls it without passing any argument:

const mnemonic = [
  "Please",
  "Send",
  "Cats",
  "Monkeys",
  "And",
  "Zebras",
  "In",
  "Large",
  "Cages",
  "Make",
  "Sure",
  "Padlocked",
];

// Return value
console.log(mnemonic.splice()); // []

console.log(mnemonic); // ["Please", "Send", "Cats", "Monkeys", "And", "Zebras", "In", "Large", "Cages", "Make", "Sure", "Padlocked"]

As we can see, splice() without argument returns an empty array []. And as we'll see it become clearer in the next section, it is an array of removed items. In the case above, since no item is being removed, it is empty.

Notice that, although splice() is geared to remove items, it did not mutate the caller above, since we have not passed startIndex.

Wiping All After n Items - JavaScript splice() with Only startIndex​

When we pass startIndex, we should expect the caller array to mutate.

With only startIndex passed to JavaScript splice(), all items including the one at and after startIndex are removed:

const mnemonic = [
  "Please",
  "Send",
  "Cats",
  "Monkeys",
  "And",
  "Zebras",
  "In",
  "Large",
  "Cages",
  "Make",
  "Sure",
  "Padlocked",
];

// Return value
console.log(mnemonic.splice(4)); // ["And", "Zebras", "In", "Large", "Cages", "Make", "Sure", "Padlocked"]

// Caller array
console.log(mnemonic); // ["Please", "Send", "Cats", "Monkeys"]

We can clearly see from this example that the returned array now has items including the one at startIndex (4) till the end removed. In other words, they were wiped out and the items upto startIndex are remaining in the original array.

It is important to notice that removal begins at startIndex included. This gives rise to a noticeable pattern where the zero based startIndex actually corresponds to first n items remaining in the caller array, i.e. n = startIndex:

const mnemonic = [
  "Please",
  "Send",
  "Cats",
  "Monkeys",
  "And",
  "Zebras",
  "In",
  "Large",
  "Cages",
  "Make",
  "Sure",
  "Padlocked",
];

// Wiping all after first 4 items
console.log(mnemonic.splice(4)); // ["And", "Zebras", "In", "Large", "Cages", "Make", "Sure", "Padlocked"]
console.log(mnemonic); // ["Please", "Send", "Cats", "Monkeys"]

// Wiping all after first 2 items
console.log(mnemonic.splice(2)); // ["Cats", "Monkeys"]
console.log(mnemonic); // ["Please", "Send"]

// Wiping all items
console.log(mnemonic.splice(0)); // ["Please", "Send"]
console.log(mnemonic); // []

In the above examples, for each of the values passed as startIndex, we want that number of items to remain at the head of the original array. With mnemonic.splice(0), we are emptying the original array.

Deleting Intermediary Items with JS Array.prototype.splice()​

We can remove intermediary items from an array by passing deleteCount as the second argument. This removes the indicated number of items beginning at startIndex:

const mnemonic = [
  "Please",
  "Send",
  "Cats",
  "Monkeys",
  "And",
  "Zebras",
  "In",
  "Large",
  "Cages",
  "Make",
  "Sure",
  "Padlocked",
];
console.log(mnemonic.splice(4, 2)); // ["And", "Zebras"]
console.log(mnemonic); // ["Please", "Send", "Cats", "Monkeys", "In", "Large", "Cages", "Make", "Sure", "Padlocked"]

In the above call, we are passing 2 as deleteCount. Deleting starts at startIndex and ends at the index where deleteCount is achieved. Notice, removal slides the items after deleteCount to the left.

Replacing Items with JS Array.prototype.splice()​

We can replace removed items with new ones by passing in items as a list after the second argument. All items passed are added to startIndex onwards:

const mnemonic = [
  "Please",
  "Send",
  "Cats",
  "Monkeys",
  "And",
  "Zebras",
  "In",
  "Large",
  "Cages",
  "Make",
  "Sure",
  "Padlocked",
];
console.log(mnemonic.splice(4, 0, "Ants", "Zebus")); // []
console.log(mnemonic); // ["Please", "Send", "Cats", "Monkeys", "Ants", "Zebus", "And", "Zebras", "In", "Large", "Cages", "Make", "Sure", "Padlocked"]
console.log(mnemonic.splice(4, 2)); // ["Ants", "Zebus"]
console.log(mnemonic); // ["Please", "Send", "Cats", "Monkeys", "And", "Zebras", "In", "Large", "Cages", "Make", "Sure", "Padlocked"]

// Items removed first and then added
console.log(mnemonic.splice(4, 2, "Alpacas", "Zorillas"));
console.log(mnemonic); // ["Please", "Send", "Cats", "Monkeys", "Alpacas", "Zorillas", "In", "Large", "Cages", "Make", "Sure", "Padlocked"]

Observe above that deletion gains precedence over insertion. It becomes more apparent when deleteCount is positive and items are replaced or added with arguments after . In the above series of mutations,"Ants", "Zebus" first inserted at 4, with 0 items removed first. Then they were removed with 4 as startIndex, 2 as deleteCount and no items replacing them. In the last mutation, further removal of 2 items at index 4 happens, that are replaced subsequently with the new items.

We can actually go further and overload the array by adding more items than removed:

const mnemonic = [
  "Please",
  "Send",
  "Cats",
  "Monkeys",
  "And",
  "Zebras",
  "In",
  "Large",
  "Cages",
  "Make",
  "Sure",
  "Padlocked",
];

// Add more items than removed
console.log(mnemonic.splice(4, 2, "Ants", "Zebus", "Donkeys", "Bears")); // ["And", "Zebras"]
console.log(mnemonic); // ["Please", "Send", "Cats", "Monkeys", "Ants", "Zebus", "Donkeys", "Bears", "In", "Large", "Cages", "Make", "Sure", "Padlocked"]

Notice that because we are replacing or overloading items, the items that come after get displaced towards the right.

Items Injection with Array.prototype.splice()​

You might already noticed that, if we want to only inject items at a certain point in the array, we can pass 0 as deleteCount:

const mnemonic = [
  "Please",
  "Send",
  "Cats",
  "Monkeys",
  "And",
  "Zebras",
  "In",
  "Large",
  "Cages",
  "Make",
  "Sure",
  "Padlocked",
];
console.log(mnemonic.splice(4, 0, "Ants", "Zebus", "Donkeys", "Bears")); // []
console.log(mnemonic); // ["Please", "Send", "Cats", "Monkeys", "Ants", "Zebus", "Donkeys", "Bears", "And", "Zebras", "In", "Large", "Cages", "Make", "Sure", "Padlocked"]

New items are added starting at startIndex directly because with deleteCount set to 0, so no items are deleted.

Tailgrowing with Array.prototype.splice()​

We can add to the tail by using the length of the caller array as startIndex:

const mnemonic = [
  "Please",
  "Send",
  "Cats",
  "Monkeys",
  "And",
  "Zebras",
  "In",
  "Large",
  "Cages",
  "Make",
  "Sure",
  "Padlocked",
];
console.log(
  mnemonic.splice(mnemonic.length, 0, "Ants", "Zebus", "Donkeys", "Bears"),
); // []
console.log(mnemonic); // ["Please", "Send", "Cats", "Monkeys", "In", "Large", "Cages", "Make", "Sure", "Padlocked", "Ants", "Zebus", "Donkeys", "Bears"]

Caller length finds the tail for an incoming item as length is greater than the last index in the zero-index based array. Using length like this with splice() is a good occasional alternative to Array.prototype.push().

It important to caution that using -1 as startIndex doesn't add to the tail. That's because for startIndex < 0, mutation starts at startIndex + mnemonic.length. So, the last item always gets displaced as last and we never add to the tail:

const mnemonic = [
  "Please",
  "Send",
  "Cats",
  "Monkeys",
  "And",
  "Zebras",
  "In",
  "Large",
  "Cages",
  "Make",
  "Sure",
  "Padlocked",
];
console.log(mnemonic.splice(-1, 0, "Ants", "Zebus", "Donkeys", "Bears")); // []
console.log(mnemonic); // ["Please", "Send", "Cats", "Monkeys", "And", "Zebras", "In", "Large", "Cages", "Make", "Sure" "Ants", "Zebus", "Donkeys", "Bears", "Padlocked"]

A variation of this next.

Inject Items Before Last n Items - JavaScript slice() with Negative startIndex​

We can inject new items before last n items by passing in -n as startIndex. This is another example when startIndex < 0:

const mnemonic = [
  "Please",
  "Send",
  "Cats",
  "Monkeys",
  "And",
  "Zebras",
  "In",
  "Large",
  "Cages",
  "Make",
  "Sure",
  "Padlocked",
];

// Inject before last 2 items
console.log(mnemonic.splice(-2, 0, "Ants", "Zebus", "Donkeys", "Bears")); // []
console.log(mnemonic); // ["Please", "Send", "Cats", "Monkeys", "And", "Zebras", "In", "Large", "Cages", "Make", "Ants", "Zebus", "Donkeys", "Bears", "Sure", "Padlocked"]

// Inject before last 4 items
console.log(mnemonic.splice(-4, 0, "Apacas", "Zorillas")); // []
console.log(mnemonic); // ["Please", "Send", "Cats", "Monkeys", "And", "Zebras", "In", "Large", "Cages", "Make", "Ants", "Zebus", "Apacas", "Zorillas", "Donkeys", "Bears", "Sure", "Padlocked"]

Again, a negative startIndex brings -n + mnemonic.length to play. So, mutation starts at n items (not indexes) from the end of the array -- keeping last n items intact in the tail.

Nuances of JS Array.prototype.splice()​

In this section, let's consider other quirks of using JavaScript slice().

Higher Negatives of startIndex​

There is another nuance of using negative startIndex with splice(). For example, when startIndex < -mnemonic.length, it is effectively 0:

const mnemonic = [
  "Please",
  "Send",
  "Cats",
  "Monkeys",
  "And",
  "Zebras",
  "In",
  "Large",
  "Cages",
  "Make",
  "Sure",
  "Padlocked",
];
console.log(mnemonic.splice(-100, 10, "Ants", "Zebus", "Donkeys", "Bears")); // ["Please", "Send", "Cats", "Monkeys", "And", "Zebras", "In", "Large", "Cages", "Make"]
console.log(mnemonic); // ["Ants", "Zebus", "Donkeys", "Bears", "Sure", "Padlocked"]

Here, with startIndex value of -100, removal starts at index 0 and first 10 items are removed. The passed items are added after removal at index 0.

Higher startIndex Value​

When startIndex is greater than the length of the array, no items is deleted even if deleteCount is passed. Plus all passed items are tucked to the tail:

const mnemonic = [
  "Please",
  "Send",
  "Cats",
  "Monkeys",
  "And",
  "Zebras",
  "In",
  "Large",
  "Cages",
  "Make",
  "Sure",
  "Padlocked",
];
console.log(mnemonic.splice(100, 10, "Ants", "Zebus", "Donkeys", "Bears")); // []
console.log(mnemonic); // ["Please", "Send", "Cats", "Monkeys", "And", "Zebras", "In", "Large", "Cages", "Make", "Sure", "Padlocked", "Ants", "Zebus", "Donkeys", "Bears"]

As we can see, 100 is far greater than mnemonic.length, and we asked for deletion of 10 items somewhere non-existent. So, none is deleted inside the array. And passed items are added to the tail.

Using an arbitrary higher index is, however, not safe for tailgrowing, since it does not always make sure startIndex is greater or equal to caller length.

Bonus: Splice vs. Slice in JavaScript​

I will briefly explain the difference between two methods in JavaScript: splice() and slice(). Generally, they are mistaken for being similar methods. The reason is that both these methods bear the same name.

splice()​

• Mutates the original array.
• Can be used to remove, replace, or insert elements at a specific index.
• It mutates the array in place and returns an array of removed elements.

Sample:

let arr = [1, 2, 3, 4];
let removed = arr.splice(2, 2); // Starting from index 2, this will remove 2 elements
console.log(arr); // [1, 2, 5] - The original array is mutated
console.log(removed); // [3, 4] - The removed elements

slice()​

• Does not modify the original array.
• It returns the shallow copy of a portion of an array from the start index to the end index, one level deep.
• The original array is not modified.

Example:

let arr = [1, 2, 3, 4];
let sliced = arr.slice(2, 4); // Copies elements from index 2 to 4 (excluding 4)
console.log(arr); // [1, 2, 3, 4, 5] - The original array is not changed
console.log(sliced); // [3, 4] - The sliced part

Key Differences:​

• splice() is a method that modifies the original array, whereas slice() doesn't.

Use the method splice() when you want to alter an array by removing, inserting, or replacing elements and use the method slice() when you want to retrieve a part of the array without having to change the array.

Splice on Multidimensional Arrays​

Here, I would like to share some insights about how the splice() method works in multidimensional arrays with JavaScript. If you have an array that contains other arrays, then it allows for changes through the splice() of the outer and inner arrays, respectively.

Modifying the Outer Array​

You can use splice() to add or remove entire inner arrays from the outer array:

let matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9],
];

To pop out the second row (i.e., the array [4, 5, 6]):

let removedRow = matrix.splice(1, 1); // Remove 1 element at index 1
console.log(matrix);
// Output: [[1, 2, 3], [7, 8, 9]]

console.log(removedRow);
// Output: [[4, 5, 6]]

Changing Inner Arrays​

You can also splice() the elements inside:

// Delete the second element of the first row
let removedElement = matrix[0].splice(1, 1); // Removes 1 element at index 1 of the first inner array
console.log(matrix);
// Output: [[1, 3], [7, 8, 9]]

console.log(removedElement);
// Output: [2]

Inserting Elements​

Similarly, you can push elements into inner arrays:

// Populate the second row
matrix[1].splice(2, 0, 10, 11);
// Pushes 10 and 11 into the second inner array at index 2

console.log(matrix);
// Output: [[1, 3], [7, 8, 10, 11, 9]]

Application Example​

Let's assume we have a multidimensional array representation of a schedule, where each inner array represents the tasks to be completed that day:

let weeklySchedule = [
  ["Meeting", "Coding", "Review"], // Monday
  ["Planning", "Design"], // Tuesday
  ["Deployment", "Testing"], // Wednesday
];

If we have to scratch off "Design" on the schedule for Tuesday:

weeklySchedule[1].splice(1, 1); // Removes 1 element at index 1 of Tuesday's array

console.log(weeklySchedule);
// Output: [["Meeting", "Coding", "Review"], ["Planning"], ["Deployment", "Testing"]]

Key Points​

• Accessing Inner Arrays: Access inner arrays using array indices before splicing.
• Replacing Inner Array Elements: The splice() function can be used to add, remove, or replace elements from the inner arrays.
• Changing the Outer Array: The splice() method is capable of adding or removing complete arrays that are enclosed within the outer array.

Using Splice with Destructuring in JavaScript​

Here's a cool trick in JavaScript: how to use splice() with destructuring. It can really simplify lots of things when you want to manipulate an array, yet also makes the code much more readable as you're working with specific elements within that array.

Basic Splice + Destructuring​

We can use splice() in an array to remove elements. Using destructuring, we are able to directly assign the removed items to variables.
Here is an example to destructure and remove:

let arr = [1, 2, 3, 4, 5];

// Splice the first two away, and destructure the result
let [first, second] = arr.splice(0, 2);

console.log(first); // Output: 1
console.log(second); // Output: 2
console.log(arr); // Output: [3, 4, 5] - The original array is now modified

Here, splice() removes the first two elements, destructuring assigns them to first and second, and the original array is mutated.

Example: Element Swapping​

You can also combine splice() and destructuring for more practical applications, such as swapping array elements:

let numbers = [10, 20, 30, 40, 50];

// Swap 2nd and 4th elements
[numbers[1], numbers[3]] = [numbers[3], numbers[1]];

console.log(numbers); // Output: [10, 40, 30, 20, 50]

Extracting Multiple Elements with Destructuring​

You can even extract several parts of the array using splice() and apply these immediately with destructuring:

let fruits = ["apple", "banana", "cherry", "date", "elderberry"];

// Remove 3 elements starting from index 1, destructively
const [firstFruit, secondFruit, thirdFruit] = fruits.splice(1, 3);

console.log(firstFruit); // Output: 'banana'
console.log(secondFruit); // Output: 'cherry'
console.log(thirdFruit); // Output: 'date'
console.log(fruits); // Output: ['apple', 'elderberry']

Destructuring + Insertion​

You can combine splice() with destructuring to delete and insert elements in a single shot:

let arr = [1, 2, 3, 4, 5];

// Destructure two elements from the index 2 of the array
let [removed1, removed2] = arr.splice(2, 2, "a", "b");

console.log(removed1); // Output: 3
console.log(removed2); // Output: 4
console.log(arr); // Output: [1, 2, 'a', 'b', 5] - Array is modified with new elements

In this example, we delete elements 3 and 4 and then insert 'a' and 'b' into their places, while destructuring the deleted items into variables removed1 and removed2.

Why Use This?​

• Cleaner Code: Use destructuring with splice() to avoid writing many lines of code while removing elements from it.
• Direct Access: The removed elements are available immediately for working, and the array is ready to be manipulated in a single step.

Summary​

In this post, explored various cases and patterns of using JavaScript splice(). We learned that Array.prototype.splice() is an array method and it doesn't have a String equivalent. We also understood that it mutates the original array and returns an array of removed items.

We found out how to wipe out all items after first n items by passing n as the startIndex to splice(). We illustrated how to remove intermediary items by passing deleteCount as the second argument. We also saw how to replace removed items by passing in new items as third argument and onward. We learned how to inject items with 0 passed as deleteCount.

We figured out an alternative to Array.prototype.push() by using Array.prototype.splice() with caller length passed as startIndex. We also discovered the pattern of adding items prior to last n items using -n as the startIndex. In the end, we explored the quirks of using extreme values of startIndex, particularly when it is more negative than -length of the caller array and when it is higher than length.