How To Strongly Type process.env
Learn how to strongly type process.env in TypeScript by either augmenting global type or validating it at runtime with t3-env.
Cannot redeclare block-scoped variable 'name'.
This error is likely occurring because you have two variables with the same name, or you might have tried to modify something in the global scope.
There are a few main solutions, based on the type of problem you're having:
If you have two variables with the same name in the same scope, it'll cause this error:
ts
letCannot redeclare block-scoped variable 'id'.2451Cannot redeclare block-scoped variable 'id'.= 123; id letCannot redeclare block-scoped variable 'id'.2451Cannot redeclare block-scoped variable 'id'.= 456; id
This is because when a variable is declared, its value is saved in memory. If you try to declare another variable with the same name in the same scope, it'll try to save it in the same place, which will cause an error.
So, the simplest fix is to rename one of the variables.
If you want to retain the same name, you can change the scope of one of the variables:
ts
letid = 123;{letid = 456;console .log (id ); // 456}console .log (id ); // 123
This works because the two variables are in different scopes. The first id
is in the scope of the module, and the second id
is in the scope of the block.
But, as you can see, this can be confusing for folks reading your code - you should avoid naming variables the same thing in different scopes.
If you don't have two variables with the same name, it's possible that you've accidentally tried modifying the global scope.
Let's take a look at this classic error:
Cannot redeclare block-scoped variable 'name'.
ts
constCannot redeclare block-scoped variable 'name'.2451Cannot redeclare block-scoped variable 'name'.= "Matt"; name
We've only got one name
in our file, so what's going on?
This is happening for two reasons. First, our file doesn't have any imports or exports in it. If we add an empty export, the error goes away:
ts
constname = "Matt";export {};
When TypeScript doesn't see an import or export, it considers the file to be a script, not a module.
Scripts are loaded into the global scope of browsers via the <script />
tag - so any code we write here is at the same scope as globals like window
and document
.
And, as it turns out, name
is one of these global variables. So, when we try to declare a variable with the same name, TypeScript throws an error.
moduleDetection
to force
If you're in a project where every file is a module, not a script, you should change moduleDetection
to force
in your tsconfig.json
.
This will make TypeScript treat every file as a module, even if it doesn't have any imports or exports.
json
{"compilerOptions": {"moduleDetection": "force"}}
Long-term, this is the best way to avoid this error.
Share this article with your friends
Learn how to strongly type process.env in TypeScript by either augmenting global type or validating it at runtime with t3-env.
Discover when it's appropriate to use TypeScript's any
type despite its risks. Learn about legitimate cases where any
is necessary.
Learn why TypeScript's types don't exist at runtime. Discover how TypeScript compiles down to JavaScript and how it differs from other strongly-typed languages.
Improve React TypeScript performance by replacing type & with interface extends. Boost IDE and tsc speed significantly.
In this book teaser, we discuss deriving vs decoupling your types: when building relationships between your types or segregating them makes sense.
Learn how TypeScript's new utility type, NoInfer, can improve inference behavior by controlling where types are inferred in generic functions.