Agent Skills: Stop Stuffing Workflows Into Your Rules File
Salatiel Lima | Apr 29, 2026
Summary
- The article demonstrates how to add, edit, and remove items in JavaScript arrays using both mutating and non-mutating approaches.
- Mutating examples use methods like push, unshift, splice, pop, and shift, along with direct index assignment for editing.
- Non-mutating examples rely on slice, concat, map, filter, and spread syntax to return a new array reference instead of altering the original.
- Avoiding mutation is presented as a way to write safer code, reduce bugs, and support patterns like unidirectional data flow and easier data tracking.
Originally posted on dev.to
In this article, I’ll focus on showing how to add, edit and remove items in an array causing mutation and non-mutation ways.
One thing we need to keep in mind when writing code avoiding mutation is to return a new reference to the data after the update.
It’s a common approach when working with functional programming and if you want to understand some concepts of functional programming I recommend you read this article I wrote some time ago.
Why Avoid Mutation
When you work with immutable data you can have some positive impacts like the following:
– Tracking data without mutation is much better;
– Immutable states help you implement unidirectional data flow that helps you handle data;
I really recommend you read this article to go deeper into why avoid mutation.
Causing Mutation
The following steps will cause mutation into the array when adding, removing and editing elements from `family`.
To show an example of mutating, we’ll use the following array:
const heroesMutate = ['Spider-man', 'Thor', 'Hulk', 'Iron Man'];
console.log(heroesMutate); // => ["Spider-man", "Thor", "Hulk", "Iron Man"]
Including in Array
Methods that will be used:
See the following use-case examples for these methods:
heroesMutate.push('Captain Marvel');
console.log(heroesMutate); // => ["Spider-man", "Thor", "Hulk", "Iron Man", "Captain Marvel"]
heroesMutate.unshift('Deadpool');
console.log(heroesMutate); // => ["Deadpool", "Spider-man", "Thor", "Hulk", "Iron Man", "Captain Marvel"]
heroesMutate.splice(2, 0, 'Black Panther');
console.log(heroesMutate); // => ["Deadpool", "Spider-man", "Black Panther", "Thor", "Hulk", "Iron Man", "Captain Marvel"]
Editing the Array
The following case will find index for the element we want to edit and set value to the index found:
const indexDeadpool = heroesMutate.indexOf('Deadpool');
heroesMutate[indexDeadpool] = 'Wolverine';
console.log(heroesMutate); // => ["Wolverine", "Spider-man", "Black Panther", "Thor", "Hulk", "Iron Man", "Captain Marvel"]
Removing in the Array
Methods that will be used:
See the following use-case examples for these methods:
heroesMutate.pop();
console.log(heroesMutate); // => ["Wolverine", "Spider-man", "Black Panther", "Thor", "Hulk", "Iron Man"]
heroesMutate.shift();
console.log(heroesMutate); // => ["Spider-man", "Black Panther", "Thor", "Hulk", "Iron Man"]
heroesMutate.splice(1, 1);
console.log(heroesMutate); // => ["Spider-man", "Thor", "Hulk", "Iron Man"]
Avoiding Mutation
In this topic, we’ll add, remove and edit, avoiding mutations.
Methods that will be used:
- Array.prototype.slice();
- Array.prototype.concat();
- Array.prototype.map();
- Array.prototype.filter();
- Spread syntax;
See the following use-cases:
const villains = ['Loki', 'Thanos', 'Venom', 'Abomination'];
Including in the Array
Add to the end of array:
const newVillains = villains.concat('Juggernaut');
const newVillains2 = [...newVillains, 'Magneto'];
const newVillains3 = ['Red Skull', ...newVillains2];
console.log(villains); // => ["Loki", "Thanos", "Venom", "Abomination"]
console.log(newVillains); // => ["Loki", "Thanos", "Venom", "Abomination", "Juggernaut"]
console.log(newVillains2); // => ["Loki", "Thanos", "Venom", "Abomination", "Juggernaut", "Magneto"]
console.log(newVillains3); // => ["Red Skull", "Loki", "Thanos", "Venom", "Abomination", "Juggernaut", "Magneto"]
In the following example we’ll add Ultron after Thanos in the array:
const newVillains = [...villains.slice(0, 2), 'Ultron', ...villains.slice(2, villains.length)];
console.log(villains); // => ["Loki", "Thanos", "Venom", "Abomination"]
console.log(newVillains); // => ["Loki", "Thanos", "Ultron", "Venom", "Abomination"]
Editing the Array
In the following example we’ll edit `Venom` to `Galactus`:
const indexVenom = villains.indexOf('Venom');
const newVillains = [...villains.slice(0, indexVenom), 'Galactus', ...villains.slice(indexVenom+1)];
const newVillains2 = newVillains.map(v => v === 'Abomination' ? 'Ultron' : v);
console.log(villains); // => ["Loki", "Thanos", "Venom", "Abomination"]
console.log(newVillains); // => ["Loki", "Thanos", "Galactus", "Abomination"]
console.log(newVillains2); // => ["Loki", "Thanos", "Galactus", "Ultron"]
Removing in the Array
In the following example we’ll remove Thanos from the array:
const indexThanos = villains.indexOf('Thanos');
const newVillains = [...villains.slice(0, indexHelder), ...villains.slice(indexHelder+1)];
const newVillains2 = newVillains.filter(v => v !== 'Thanos');
console.log(villains); // => ["Loki", "Thanos", "Venom", "Abomination"]
console.log(newVillains); // => ["Loki", "Venom", "Abomination"]
console.log(newVillains2); // => ["Loki", "Abomination"]
See that in all the examples that we developed above, a new instance of the array was created, thus avoiding the mutation of the initially defined arrays.
Wrapping Up
Avoiding mutations is a safe and one-way path.
When you realize that you’re writing code observing this type of detail, believe me, you will be writing better, secure code and avoiding possible bugs due to mutation.
Feel free to share your feedback and experience in the comments.
Enjoy programming! ✨
References
FAQ
Why should mutation be avoided when working with arrays?
Avoiding mutation offers positive impacts such as better data tracking without mutation, and immutable states that help implement unidirectional data flow, which makes handling data easier.
Which array methods cause mutation when adding, editing, or removing elements?
Methods that cause mutation include Array.prototype.push(), Array.prototype.unshift(), and Array.prototype.splice() for adding; direct index assignment for editing; and Array.prototype.pop(), Array.prototype.shift(), and Array.prototype.splice() for removing.
Which methods can be used to add, edit, or remove array elements without causing mutation?
To avoid mutation, you can use Array.prototype.slice(), Array.prototype.concat(), Array.prototype.map(), Array.prototype.filter(), and the spread syntax.
What is the key principle when writing code that avoids mutation?
The key principle is to return a new reference to the data after the update. This is a common approach in functional programming, ensuring that a new instance of the array is created instead of modifying the original.
How can you add an element at a specific position in an array without mutating it?
You can use the spread syntax combined with slice(). For example, to add 'Ultron' after 'Thanos': const newVillains = [...villains.slice(0, 2), 'Ultron', ...villains.slice(2, villains.length)]; This creates a new array without modifying the original.
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