Methods are fns which can be called on objects
Arrays are objects, hence they also have methods in JS.

slice(begin): extract part of array in a new array, without mutating the original array.

let arr = ['a','b','c','d','e'];

// Usecase: Extract till index passed as argument
arr.slice(2); // [ 'c', 'd', 'e' ]

// Usecase: Extract from [first index] to [second index-1] value.
arr.slice(2,4); // [ 'c', 'd' ] i.e Length of array will be end-begin or 4-2 = 2 

// Usecase: Extract last 2 elements
arr.slice(-2); // [ 'd', 'e' ]

// Usecase: Extract the last element.
arr.slice(-1);  // [ 'e' ]  

// Usecase: Extract from index passed to leaving the last two elements.
arr.slice(1,-2);  // [ 'e' ]  

// Usecase: Create a shallow copy of an array
arr.slice(); // 1st way
[...arr]; // 2nd way

splice: mutates the original array

// splice: remove the elements begining from the index passed. Mutates the orig. array.
// returns: part of the removed array
let arr = ['a','b','c','d','e'];
// arr.splice(2); // [ 'c', 'd', 'e' ]
// arr; // [ 'a', 'b' ]

// Usecase: Remove last element of the array
// arr.splice(-1); // [ 'e' ]
// arr; // [ 'a', 'b', 'c', 'd' ]

// Usecase: Delete no of elements. splice(index, deleteCount)
arr.splice(1, 3); // [ 'b', 'c', 'd' ]
arr; // [ 'a', 'e' ]

reverse: mutates the original array.

returns: reversed array

let arr = ['a','b','c','d','e'];
let arr2 = arr.reverse();
arr;
arr2;

concat: joins two arrays.

returns: joined array

let arr1 = ['a','b','c','d','e'];
let arr2 = ['f','g','h','i','j'];

[...arr1, ...arr2];       // 1st way
arr2 = arr1.concat(arr2); // 2nd way

join: joins two arrays.

returns: joined array

let arr1 = ['a','b','c','d','e'];
let arr2 = ['f','g','h','i','j'];
const x = arr1.concat(arr2);
x.join('-'); // 'a-b-c-d-e-f-g-h-i-j'

"As time progresses, you'll remember them based on their usage."

at: starts counting from the end, begining with index as -1

Suports method chaining. Works on arrays, strings

let arr = ['a','b','c','d','e'];

arr[0];    // 1st way
arr.at(0); // 2nd way

// Get the last element of the array
arr[arr.length - 1];  // 1st way
arr.slice(-1)[0];     // 2nd way
arr.at(-1);           // 3rd way

arr.at(0); // 'a'
arr.at(-1); // 'e'

forEach:

// Looping over array using forEach method.
let account = [2000,-300,+400, -200, -500, +1000, -300];

// Loop over an array using for-of
for(let money of account){
  if(money > 0){
    console.log(`Deposited ${money}`);
  } else {
    console.log(`Withdrawn ${Math.abs(money)}`);
  }
}

// .entries(): returns an array of arrays.
// return the output as index-value pair.
// first element must be index, second element must be element-value
for(let [i,money] of account.entries()){
  if(money > 0){
    console.log(`Transaction ${i+1}, Deposited ${money}`);
  } else {
    console.log(`Transaction ${i+1}, Withdrawn ${Math.abs(money)}`);
  }
}

// Loop over an array using forEach which requires a callback fn.
// forEach will call the callback fn, and not we.
// forEach will pass each element as argument in every iteration.
account.forEach(function(money){
  if(money > 0){
    console.log(`Deposited ${money}`);
  } else {
    console.log(`Withdrawn ${Math.abs(money)}`);
  }
});
// Iteration 1: pass arg1 to CB-fn(arg1)
// Iteration 2: pass arg2 to CB-fn(arg2)
// Iteration 3: pass arg3 to CB-fn(arg3)
// .....
// .....


// forEach will pass each element, index, array as argument in every iteration. Order of arguments matter, not the no of these arguments i.e first element should be the current-Element, second element should be index, third element should be entire array which is being looped-over.
// first element must be element-value, second element should be index, third element must be entire array. This is how its different from array.entries()
account.forEach(function(money, i, arr){
  if(money > 0){
    console.log(`Transaction ${i+1}, Deposited ${money} into ${arr}`);
  } else {
    console.log(`Transaction ${i+1}, Withdrawn ${Math.abs(money)} from ${arr}`);
  }
});

When to use for-of & forEach:

forEach: Cannot break-out of it. continue-break don't work inside it. Will always loop over the entire array, and nothing can be done to stop it.
for-of: Used when we need to loop out of the array.

"As you practice more & more, you'll get better in your skills."