Array Short Notes

A JavaScript array is a structured collection of values, referred to as elements, each associated with a numeric index that begins at 0. The first element resides at index 0, the second at index 1, and so forth. Arrays are highly adaptable, capable of storing diverse data types like numbers, strings, objects, or even other arrays.

Array Basics

JavaScript arrays can be created using array literals [] for simplicity and efficiency or the new Array() constructor.

Array Literal :

• Use [] for simplicity and efficiency.

let arr = []; // Empty array
let arr2 = [10, 20, 30]; // Array with mixed values

console.log(arr);
console.log(arr2);

[]
[ 10, 20, 30 ]

Array Constructor :

Use new Array().

let arr = new Array(10, 20, 30); // Array with values
console.log(arr);

[ 10, 20, 30 ]

Now we start Basic Operations on JavaScript Arrays after reading Array Basics

Basic Operations on JavaScript Arrays

JavaScript array operations include accessing elements with arr[index], accessing length with arr.length, modifying elements via assignment, adding with push() or unshift(), removing with pop(), shift(), or splice(), or combining arrays with concat() and map(), filter(), reduce().

Accessing Elements:

Use index (arr[index]).

• First element: arr[0]
• Last element: arr[arr.length - 1]

let arr = [10,20,30,40]

// Access first element
console.log(arr[0])

// Access last element
console.log(arr[arr.length-1]);

10
40

Array Length :

Use arr.length to get/set size.

let arr = [10,20,30,40];

console.log(arr.length);

4

Modifying Elements :

Assign new value to an index.

let arr = [10,20,40];

//Modifiying element
arr[2]=30;

console.log(arr);

[ 10, 20, 30 ]

Adding Element :

• push() : Add to end.
• unshift() : Add to start.

let arr = [20,40,60];

// Add Element last
arr.push(80); 

//Add Element First 
arr.unshift(0);

console.log(arr);

[ 0, 20, 40, 60, 80 ]

Removing Elements :

• pop() : Remove from end.
• shift() : Remove from start.
• splice(start, deleteCount) : Remove/replace elements from specific index.

let arr = [10,20,30,40,50,60];

//Remove End Element
arr.pop();

//Remove Start Element
arr.shift();

console.log(arr);

let arr2 = [20,40,60,80];

// Remove start to end 
arr2.splice(1,3); 

console.log(arr2);

[ 20, 30, 40, 50 ]
[ 20 ]

Concatenation:

arr1.concat(arr2) combines arrays.

let arr1 = [1,2,3,4];

let arr2 = [5,6,7,8];

//Using concat to combin arr
console.log(arr1.concat(arr2));

[
  1, 2, 3, 4,
  5, 6, 7, 8
]

Map, Filter, and Reduce :

• map(): Creates a new array by applying a function to each element.
• filter(): Creates a new array with elements that pass a test (returns true).
• reduce(): Reduces array to a single value by applying a function to each element.

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

// Map
let doubled = numbers.map(num => num * 2);
console.log(doubled); 

// Filter
let evens = numbers.filter(num => num % 2 === 0);
console.log(evens); 

// Reduce
let sum = numbers.reduce((total, num) => total + num, 0);
console.log(sum); 

[ 2, 4, 6, 8, 10 ]
[ 2, 4 ]
15

Now we start Some Problems on Array after reading Basic Operations on JavaScript Arrays.

Some Problems on Array

Array Reversal :

Reverse the order of elements in-place by swapping elements from both ends.

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

function reverseArray(arr) {
  let left = 0, right = arr.length - 1;
  while (left < right) {
    [arr[left], arr[right]] = [arr[right], arr[left]];
    left++;
    right--;
  }
  return arr;
}


console.log(reverseArray(arr));

[ 50, 40, 30, 20, 10 ]

Kadane’s Algorithm :

Find the maximum sum of a contiguous subarray in O(n) time.

let arr = [10,20,40,60,80];

function maxSubarraySum(arr) {
  let maxSoFar = arr[0], currMax = arr[0];
  for (let i = 1; i < arr.length; i++) {
    currMax = Math.max(arr[i], currMax + arr[i]);
    maxSoFar = Math.max(maxSoFar, currMax);
  }
  return maxSoFar;
}

console.log(maxSubarraySum(arr));

210