Javascript

javascript - useful techniques

1)Get the last element of array:
===============================
let numbersArr = [4, 8, 9, 34, 100];
numbersArr[numbersArr.length - 1]; //return 100

2)Get a random number in a specific range:
=========================================
// Random number between 0 and 4.
Math.floor(Math.random() * 5);
// Random number between 0 and 49.
Math.floor(Math.random() * 50);

3)Convert a multidimensional array to single dimensional:
=========================================================
let arr = [5, [1, 2], [4, 8]];
arr.flat(); //returns [5, 1, 2, 4, 8]
let twoLevelArr = [4, ["John", 7, [5, 9]]]
twoLevelArr.flat(2); //returns [4, "John", 7, 5, 9]

4)To checkout multiple conditions:
==================================
let name = "John";
//Bad way.
if(name === "John" || name === "Ben" || name === "Chris"){
  console.log("included")
}
//Better way.
if(["John", "Ben", "Chris"].includes(name)){
  console.log("included")
}

5)To get unique value in array and remove the duplicate ones:
============================================================
const languages = ['JavaScript', 'Python', 'Python', 'JavaScript', 'HTML', 'Python'];
const uniqueLanguages = [...new Set(languages)];
console.log(uniqueLanguages);
//prints: ["JavaScript", "Python", "HTML"]

6)If you want run a event only once use below:
==============================================
document.body.addEventListener('click', () => {
    console.log('Run only once');
  }, { once: true });

7)Reduce method for summing all number in an array:
===================================================
ES6-method:
let numbers = [6, 9 , 90, 120, 55];
numbers.reduce((a, b)=> a + b, 0); //returns 280

ES5:
var arr = [29, 46, 86.5]; 
var sum = arr.reduce( function(total, value){
  return total + value
});
console.log(sum); // 161.5

reduce()method will iterate all the element in an array. 
total it is intila value or previuosly retruned value of the function.
value is the next vlaue in the array.

8)In operator:
===============
Used to check a property is defined in an obejct or an element is included inside an array.
const employee = {
  name: "Chris",
  age: 25
}
"name" in employee; //returns true.
"age" in employee;  //returns true.
"experience" in employee; //retuens false.