Joanne's Blog

不可不知的 JavaScript 基本觀念

Joanne -

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

Operator Description
== equal value
=== equal value and type
!= not equal value
!== not equal value or type

Reference