Let's revisit the three keywords we can use to declare variables: var
, let
, and const
.
var |
let |
const |
|
---|---|---|---|
Variable can be re-assigned? | yes | yes | no |
Variable is limited to the block where it was defined? | no | yes | yes |
Both let
and const
have advantages over var
, but var
has no advantage over either of them.
We use let
to define variables that will change. For example, if we plan to do math on a variable, we should use let
so we can change its value.
We use const
to define variables that won't change. This helps keep our code safe because if we accidentally try to change the value, JavaScript will throw an error to show us our mistake. It is better to discover mistakes early so we can fix them as soon as possible.
Both let
and const
create variables that can only be accessed within the block where they were defined.
Summary
The general rule is to use const
whenever you can, but if you need to change the variable use let
instead.
Since we have let
and const
, we don't have a good reason to use the var
keyword anymore.