For loop in JS

// output: John, Mathew, Luke, Mark, Abraham, Issac

Usual way:

const names = ['John', 'Mathew', 'Luke', 'Mark', 'Abraham', 'Issac'];

for (let index = 0; index < names.length; index++) {
  const element = names[index];
  console.log(element);
}

Simplified:

const names = ['John', 'Mathew', 'Luke', 'Mark', 'Abraham', 'Issac'];

// get values in index
for (let ele of names) {
  console.log(ele);
}

// get index
for (let ele in names) {
  console.log(ele);
}

forEach in JS

Note: forEach dont return anything, it will be undefined

We can use forEach concept too for above example: I have divided into 2 ways

defined function

const names = ['John', 'Mathew', 'Luke', 'Mark', 'Abraham', 'Issac'];

function eachName(item) {
  console.log(item);
}

names.forEach(eachName);

anonymous function

const names = ['John', 'Mathew', 'Luke', 'Mark', 'Abraham', 'Issac'];

names.forEach(function (i) {
  console.log(i);
});

Lets see same with different input where its an array of objects, I will use here the anonymous function (can be achieved with defined function too)

const people = [
  { uName: 'John', role: 'SDE' },
  { uName: 'Mathew', role: 'SDE2' },
  { uName: 'Luke', role: 'SDE4' },
  { uName: 'Mark', role: 'TL' },
  { uName: 'Abraham', role: 'Principal Staff' },
  { uName: 'Issac', role: 'MD' },
];

people.forEach(function (eachPerson) {
  console.log(eachPerson.uName);
});

map in JS

From above example of people, we want to get an array of roles available.

How to achieve this, we can use map here, it return new array

const people = [
  { uName: 'John', role: 'SDE' },
  { uName: 'Mathew', role: 'SDE2' },
  { uName: 'Luke', role: 'SDE4' },
  { uName: 'Mark', role: 'TL' },
  { uName: 'Abraham', role: 'Principal Staff' },
  { uName: 'Issac', role: 'MD' },
];

const roles = people.map(function (item) {
  return item.role;
});

console.log('New Array:', roles);

// output: ["SDE", "SDE2", "SDE4", "TL", "Principal Staff", "MD"]

filter in JS

similar to map where we can get new array, but for certain condition alone. Then we can use filter

const people = [
  { uName: 'John', role: 'SDE', age: 25 },
  { uName: 'Mathew', role: 'SDE2', age: 25 },
  { uName: 'Luke', role: 'SDE4', age: 30 },
  { uName: 'Mark', role: 'TL', age: 35 },
  { uName: 'Abraham', role: 'Principal Staff', age: 36 },
  { uName: 'Issac', role: 'MD', age: 40 },
];

const youngForce = people.filter(function (item) {
  return item.age <= 30;
});

console.log('Young ppl', youngForce);

//output:
[
    {
        "uName": "John",
        "role": "SDE",
        "age": 25
    },
    {
        "uName": "Mathew",
        "role": "SDE2",
        "age": 25
    },
    {
        "uName": "Luke",
        "role": "SDE4",
        "age": 30
    }
]

find in JS

Similar to filter, but find returns one instance, first match value even if multiple values exist. Good for unique values. returns undefined if no match found

const people = [
  { uName: 'John', role: 'SDE', age: 25, id: 1 },
  { uName: 'Mathew', role: 'SDE2', age: 25, id: 2 },
  { uName: 'Luke', role: 'SDE4', age: 30, id: 3 },
  { uName: 'Mark', role: 'TL', age: 35, id: 4 },
  { uName: 'Abraham', role: 'Principal Staff', age: 36, id: 5 },
  { uName: 'Issac', role: 'MD', age: 40, id: 5 },
];

const unique = people.find(function (item) {
  return item.id === 5;
});

console.log('unique', unique);

//output: return first match ie Abraham
{
    "uName": "Abraham",
    "role": "Principal Staff",
    "age": 36,
    "id": 5
}

Find Duplicate/Unique in Array

const names= ['John', 'Mathew', 'John', 'Issac', 'Abraham', 'Issac'];

const toFindDuplicates = arry => arry.filter((item, index) => arry .indexOf(item) !== index)
console.log(toFindDuplicates (names))

const toFindUnique = arry => arry.filter((item, index) => arry .indexOf(item) === index)
console.log(toFindUnique(names))