Data structures

Summary of fundamental concepts:

Structure	Key characteristics	Code example	Allowed keys	Preferred use cases	Key differences
Object	- Stores key-value pairs. - Keys can only be strings or symbols. - Does not maintain insertion order (except for numbers).	js const obj = { key: "value", 42: "number key" }; console.log(obj.key);	Strings, Symbols	When keys are known strings in advance and fast access is needed.	Not iterable with for...of. Accessed with obj.key or obj["key"].
Array	- Ordered collection of values. - Accessible by numeric index. - Useful methods like .push(), .map(), .filter().	js const arr = [1, 2, 3]; console.log(arr[0]);	Numeric indices	Ordered lists of elements when order and position matter.	Maintains insertion order. Iterable with for...of.
Map	- Key-value pairs, but keys can be of any type. - Maintains insertion order.	js const map = new Map(); map.set("key", 42); console.log(map.get("key"));	Any data type (including objects and functions)	When search efficiency is needed and keys can be of any type.	Better performance than Object for large data sets.
Set	- Collection of unique values (not key-value pairs). - Does not allow duplicates. - Maintains insertion order.	js const set = new Set([1, 2, 3, 3]); console.log(set.has(2));	No keys, only unique values.	When a list without duplicates is needed or to quickly check if a value is present.	Does not allow index access like an Array.

Methods .map, .filter, .forEach

Method	Description	Code example	Return value	Use cases	Key differences
.map()	Creates a new array by applying a function to each element of the original array.	js const nums = [1, 2, 3]; const doubled = nums.map(n => n * 2); console.log(doubled); // [2, 4, 6]	New transformed array	When each element needs to be transformed and a new array returned.	Does not modify the original array. Always returns a new array.
.filter()	Creates a new array with elements that meet a condition.	js const nums = [1, 2, 3, 4]; const evens = nums.filter(n => n % 2 === 0); console.log(evens); // [2, 4]	New array with filtered elements	When specific elements need to be extracted from an array.	Does not modify the original array. Returns only elements that meet the condition.
.forEach()	Executes a function for each element of the array without returning a new array.	js const nums = [1, 2, 3]; nums.forEach(n => console.log(n * 2));	undefined (returns nothing)	When an action needs to be performed on each element without creating a new array.	Does not return an array, only iterates over elements. Modifies the array if changes are made inside the function.

Spread and Rest Operator in JavaScript

Spread Operator (...)

The spread operator is used to make copies and combinations of arrays and objects without modifying the originals. Its main function is to expand the elements of an iterable into a new context.

Example: Copy and combine arrays

const arr1 = [1, 2, 3];
const arr2 = [...arr1, 4, 5]; // Copy and add elements
console.log(arr2); // [1, 2, 3, 4, 5]

It also works with objects:

const obj1 = { a: 1, b: 2 };
const obj2 = { ...obj1, c: 3 }; // Copy and add new properties
console.log(obj2); // { a: 1, b: 2, c: 3 }

Rest Operator (...)

The rest operator allows grouping multiple values into a single variable. It is useful in functions when the number of arguments to be received is not known in advance. It is also used in destructuring to split an array or object into parts.

Example in functions

function sum(...numbers) {
return numbers.reduce((acc, num) => acc + num, 0);
}
console.log(sum(1, 2, 3, 4)); // 10

Example in array destructuring

const [first, ...rest] = [10, 20, 30, 40];
console.log(first); // 10
console.log(rest);  // [20, 30, 40]

In objects, the rest operator allows separating some properties and grouping the rest into a new object: const { a, ...rest } = { a: 1, b: 2, c: 3 }; console.log(a); // 1 console.log(rest); // { b: 2, c: 3 }