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Array Methods

Q: Difference between slice() and splice()?

Answer: slice() is non-mutating and returns a copy of a piece of an array. splice() is mutating and changes the original array by adding or removing elements..

Q: Which methods add/remove from the START of an array?

Answer: shift() removes the first element, and unshift() adds an element to the first position..

Q: What is the result of [1, 2].concat([3, 4]) vs [...[1, 2], ...[3, 4]]?

Answer: Both result in [1, 2, 3, 4], but the spread operator approach is used more frequently in modern 2026 functional programming for its clarity and immutability..

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Expert Answer & Key Takeaways

Mastering Array Methods is essential for high-fidelity technical architecture and senior engineering roles in 2026.

JavaScript Array Methods

JavaScript provides powerful methods to transform arrays. Understanding which methods mutate (change) the original array and which return a new array is critical for high-end engineering.

1. Basic Mutators

Method	Action	Returns
`push()`	Adds element to end	New length
`pop()`	Removes last element	Removed element
`shift()`	Removes first element	Removed element
`unshift()`	Adds element to start	New length

2. The Splice Method

The splice() method can be used to add, remove, or replace elements at any position. It mutates the original array.

const fruits = ["Banana", "Orange", "Apple"];
fruits.splice(2, 0, "Lemon", "Kiwi"); // At index 2, remove 0, add two items

3. The Slice Method

The slice() method copies a section of an array into a new array. It does NOT mutate the original.

const fruits = ["Banana", "Orange", "Lemon", "Apple"];
const citrus = fruits.slice(1, 3); // Orange, Lemon

4. Joining Arrays

concat(): Merges two or more arrays. Returns a new array.
join(): Joins all array elements into a string.

const arr1 = ["Cecilie", "Lone"];
const arr2 = ["Emil", "Tobias"];
const children = arr1.concat(arr2);

[!IMPORTANT] For modern 2026 state management (like React), favor non-mutating methods like slice() or the spread operator [...] to maintain immutability.

Array Methods

Expert Answer & Key Takeaways

JavaScript Array Methods

1. Basic Mutators

2. The Splice Method

3. The Slice Method

4. Joining Arrays

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Array Methods

Expert Answer & Key Takeaways

JavaScript Array Methods

1. Basic Mutators

2. The Splice Method

3. The Slice Method

4. Joining Arrays

Top Interview Questions

?Interview Question

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?Interview Question

Course4All Engineering Team

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