What is “not assignable to parameter of type never” error in TypeScript?
If you have been working with TypeScript, you might have come across the error message “not assignable to parameter of type never”. This error can be quite confusing, especially for developers who are new to TypeScript. In this blog post, we will explore what this error means and how to resolve it.
The “not assignable to parameter of type never” error occurs when you try to assign a value to a parameter that has been declared with the type “never”. In TypeScript, the “never” type represents a value that will never occur. It is often used to indicate that a function will not return or that a variable cannot have any possible value.
Let’s take a look at an example to understand this error better:
function throwError(message: never): never {
throw new Error(message);
}
throwError("This is an error.");
In the above example, we have a function called “throwError” that takes a parameter of type “never”. This function is designed to throw an error with the provided message. However, when we try to call this function with a string, TypeScript throws the “not assignable to parameter of type never” error. This is because a string is not a valid value for a parameter of type “never”.
To resolve this error, we need to make sure that we are passing the correct type of value to the function. In this case, we should pass a value of type “never” to the function. Here’s an updated version of the code:
function throwError(message: never): never {
throw new Error(message);
}
throwError(undefined); // Pass a value of type "never"
In the updated code, we are passing “undefined” to the “throwError” function, which is a valid value of type “never”. This resolves the error and the code will run without any issues.
It’s important to note that the “not assignable to parameter of type never” error can also occur in other scenarios, such as when working with conditional types or union types. In such cases, the error message might be slightly different, but the underlying cause is the same – trying to assign an incompatible value to a parameter of type “never”.
To summarize, the “not assignable to parameter of type never” error in TypeScript occurs when you try to assign a value to a parameter that has been declared with the type “never”. To resolve this error, make sure that you are passing the correct type of value to the function or parameter.
We hope this blog post has helped you understand the “not assignable to parameter of type never” error in TypeScript. If you have any further questions, feel free to leave a comment below.
Leave a Reply