In my code, I have a custom type called Admin with two possible values: 'ADMIN' or 'AGENT'.
There is a function that retrieves the user role from local storage:
return localStorage.getItem('role');I am looking to verify if the retrieved item matches one of the predefined string literals mentioned earlier.
To ensure the type of a variable, create a type guard like this:
function checkIfAdmin(arg: string): arg is Admin {
return arg === 'ADMIN' || arg === 'AGENT';
}
Utilize the type guard in your code as shown below:
let role: string = localStorage.getItem('role');
if (checkIfAdmin(role)) {
... now 'role' is recognized as type Admin ...
}
A type guard function asserts the type of its argument with the returned value. By returning true, the compiler understands the specified type.
Though it may involve repeating strings and explicit tests initially, using this approach allows you to write the code once and broaden the scope of type checking effectively.
During the compilation process, types are eliminated which means you cannot rely on them for comparison purposes. Instead, consider utilizing a string-based enum like the following example:
enum UserStatus {
"ACTIVE",
"INACTIVE"
}
let status = sessionStorage.getItem("status");
if (status && status in UserStatus) {
console.log("User status is valid");
};
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