Variables vs. Let vs. Const

var

• can be redeclared

var x = "Joanne Tang";
var x = 21;

• can be used before it is declared

carName = "Benz";
var carName;

• have global scope

{
    var x = 2;
}
// x CAN be used here

• don't use it anymore

let

• cannot be redeclared

var x = "Joanne Tang";
var x = 21;
// SyntaxError: 'x' has already been declared

• must be declared before use
• can be reassigned
• have block scope

{
    let x = 2;
}
// x can NOT be used here

const

• cannot be redeclared
• must be assigned a value when they are declared

// Incorrect
const PI;
PI = 3.141592653589793;

• cannot be reassigned

const PI = 3.141592653589793;
PI = 3.14;      // This will give an error
PI = PI + 10;   // This will also give an error

• can change the elements of constant array

// You can create a constant array:
const cars = ["Benz", "Volvo", "BMW"];

// You can change an element:
cars[0] = "LEXUS";

// You can add an element:
cars.push("Audi");

const cars = ["Benz", "Volvo", "BMW"];
cars = ["LEXUS", "Volvo", "Audi"];    // ERROR

• can change the properties of constant object

// You can create a const object:
const person = {name:"Joanne Tang", age:"21", email:"joannetang@gmail.com"};

// You can change a property:
person.email = "joannetang@yahoo.com";

// You can add a property:
person.gender = "female";

const person = {name:"Joanne Tang", age:"21", email:"joannetang@gmail.com"};
person = {name:"Ben Wu", age:"25", email:"benwu@gmail.com"};    // ERROR

• have block scope

const x = 10;
// Here x is 10
{
const x = 2;
// Here x is 2
}
// Here x is 10

Comparision Operators

Reference

• https://www.w3schools.com/js/js_variables.asp
• https://www.w3schools.com/js/js_let.asp
• https://www.w3schools.com/js/js_const.asp
• https://www.w3schools.com/js/js_operators.asp