Let vs Const

In javascript, var is used to create variable. With ES6, let and const were introduced. Var still works but its recommended to use let and const. Use const to create constant value whose value never changes and use let to create variables whose value is going to change.

Arrow functions

Arrow function syntax is a bit shorter than the normal syntax since it omits the function keyword. When we use this inside an arrow function it will keep its context and not change it.

  const multiply = (number) => number * 2;

Three dots as Rest/Collector

...names in the below example is a collector which collects rest of parameters

var [city, ...names] = ["Pune", "Ashish", "Ansu", "Anju"]
console.log(city); // Pune
console.log(names); // ["Ashish", "Ansu", "Anju]

Three dots as Spread

Spread takes all elements, all properties and distributes them in new array. Spread is use to create new object to prevent reference copying as objects and arrays are reference types. So when we reassign arrays or objects, we are copying the pointer, not the value.

const names = ["Pune", "Ashish", "Ansu", "Anju"]
const updatedNames = [...names, "Patel"]
console.log(updatedNames); // [ 'Pune', 'Ashish', 'Ansu', 'Anju', 'Patel' ]

Destructuring

Destructing allows extracting array elements or object properties and store them in variable.

// Array destructuring
[firstName, lastName] = ["Ashish", "Patel"]
console.log(firstName); // Ashish
console.log(lastName); // Patel

// Object destructuring
const { firstName } = { firstName: "Ashish", lastName: "Patel"}
console.log(firstName) // Ashish