What is const in TypeScript?
In this article, we are going to see what is const
in TypeScript. So be ready to learn something new, before we have the var keyword to declare variables in TypeScript. But after the ES6 update we get two more new keywords to declare variables: let
and const
.
What is const in TypeScript?
So const
is a keyword which is come from the word constant
. Which we use to declare variables
in TypeScript. There are two other keywords that we use to use to create variables and they are var
and let
. Variables that are created using const
are constant which means we cannot re-assign
or re-declare
them after the declaration.
And even if we tried to re-assign
or re-declare
a value of a variable
created using the const
keyword we will get an error
. like below shown in the code. You can learn more const here
const birthYear:number = 2002;
console.log(birthYear); // it will log 2002 on the console.
birthYear = 2003; // it will show an error Cannot assign to 'birthYear' because it is a constant.
Code language: TypeScript (typescript)
In short, when we declare const variable
it becomes immutable
. Or it becomes a read-only
value
we can read the value but cannot update or change it. So keep in mind that use const
when you know that the value of a certain variable
will not change in your program. and use let
when you need to update or re-assign
the value of your variable
throughout your program.
Declare const in a single statement
Like the variables which are declared with var
and let
keywords. We can create the variable without giving them a value first and then when we need to add a value to them we can initialize
them where we need the value. But that can be only possible when we are using the let
, var
keywords.
consider the following code example.
var dayName:string; // Declared a variable without assigning a value to it.
dayName = 'sunday'; // assigned a value to dayName after initializing it..
console.log(dayName); // here we will get sunday log on the console.
Code language: TypeScript (typescript)
But with const
we can not do that. we have to initialize and assign a value to a declared variable right after we initialize them. Otherwise, we will get an error when we try to do that. Consider the following code example.
var monthName:string; // Missing initializer in const declaration.
Code language: TypeScript (typescript)
As I said when creating a variable with the const keyword you need to declare and initialize the variable right after you create it. So that’s what you have to consider when working with const
.
Const is block-scoped.
The const declaration
follows the same syntax as the var declaration
. Unlike variables declared with var, variables
declared with const have block scope
. This means the scope
const
declared variables
are limited to the block where they are declared for example function, if-else block, and for-loop block. And variables declared with var is a functional scope which means we can access them inside the body of our function.
Consider the following code example.
function demo() {
if(true) {
const num1:number = 22;
console.log(num1); // It will log 22 on the console.
}
console.log(num1); // It wll throw an error num1 is not defined.
}
demo();
Code language: TypeScript (typescript)
In the code example, we have a demo function
which has a if
statement inside the if block
we created a num1 variable
and assign a value
of 22 to it and we are logging the num1
variable to the console right in the if block. That is fine because of the block scope we can do that.
But then we are trying to log
the num1
variable outside of the if block
, we can’t do that because we are using const
and const
is block scoped
which means we cannot be able to use the num1 variable
outside of the if block
. That’s why we get the error
on line 6.
Conclusion
In this article, we learned about what is const in TypeScript. And some basics of, block-scope
and what you need to consider when creating variables with const
. If you don’t understand any topics or you want to learn something new then check out some pretty good courses on Codedamn. Also, check out the playground for coding practices.
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