w3School course

Course

Keyword	Description
var	Declares a variable
let	Declares a block variable
const	Declares a block constant
if	Marks a block of statements to be executed on a condition
switch	Marks a block of statements to be executed in different cases
for	Marks a block of statements to be executed in a loop
function	Declares a function
return	Exits a function
try	Implements error handling to a block of statements

• var keywords được sử dụng trong JS code từ 1995 - 2015. Hiện nay, chỉ nên dùng var khi code cần support cho các trình duyệt cũ.
• let và const được thêm vào JS code từ 2015.

Sử dụng const, let, var khi nào?

1. Always declare variables
2. Always use const if the value should not be changed
3. Always use const if the type should not be changed (Arrays and Objects)
4. Only use let if you can’t use const
5. Only use var if you MUST support old browsers.

Resources

• JavaScript Tutorial