Tell me about array methods: which ones do you use, do they mutate the array, and what do they return?

JavaScript has many methods for working with arrays. Here are the main ones that I often use:

• map(callback) — creates a new array by applying a function to each element. Does not mutate the original array.

• filter(callback) — creates a new array with elements that pass the check. Does not mutate.

• reduce(callback, initialValue) — reduces the array to a single value by applying an accumulator function. Does not mutate.

• forEach(callback) — executes a function for each element, returns undefined. Does not mutate, but you can change elements inside the function.

• push(element) — adds an element to the end of the array. Mutates the array, returns the new length.

• pop() — removes the last element. Mutates, returns the removed element.

• shift() — removes the first element. Mutates, returns the removed element.

• unshift(element) — adds an element to the beginning. Mutates, returns the new length.

• slice(start, end) — returns a new array with a copy of part of the original. Does not mutate.

• splice(start, deleteCount, ...items) — modifies the array by deleting and/or adding elements. Mutates, returns the deleted elements.

• sort() — sorts the array in place. Mutates, returns the sorted array.

• concat(array) — returns a new array by combining the original with another. Does not mutate.

Example of using map and filter:

const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(n => n * 2); // [2, 4, 6, 8, 10]
const evens = numbers.filter(n => n % 2 === 0); // [2, 4]

It is important to remember that methods that mutate the array change the original, which can lead to bugs if the state is not controlled. Methods that return a new array help write cleaner and more predictable code.