var, const or let? Which one to use?

Short version:

When should I use:

• var : never again
• const : whenever possible
• let : when the variable needs to be overwritten

Long version:

Version 6 of ECMAScript brought with it new commands to declare variables, let and const . This came to finally fix problems that var had, and in practice we should not use var . It's still important to declare variables but now we have two options:

• const

The const should be used in cases where the variable does not need to be reassigned, overwritten. This is the majority of cases. The const blocks the variable to a constant value and will give error if it is reassigned, it will look like that in read-only mode, then:

const a = 10;
a = 20; // vai dar erro 

But it's worth noting that const obj = {} allows modifying the object, for example obj.foo = 'bar' but does not allow re-declaring const obj = {novo: 'objeto'} .

• let

let is the new way to declare variables that need to be overwritten. The most common case is an iteration of a loop, where the variable to iterate is overwritten N times during the loop.

Common features of let and const :

• Both let and const do not allow a variable to be re-declared. This was allowed with var , without warnings being given, creating bugs difficult to detect.

• Both declare the variable in scope and bloco where they are, not allowing within the same function or block to re-declare the same variable.

These new versions are not yet used directly in browsers because support is not yet universal, but is already used in environments pre-transpilados . var is therefore advised against new code.