"Property does not exist on type "never"

Addressing the Error: 'Property does not exist on type 'never''

So you're working on your project, feeling productive and excited, ready to tackle any challenges that come your way. But then, out of nowhere, you encounter an error message that says:

"Property 'name' does not exist on type 'never'."

Confusion sets in. What does it mean? How did 'never' get involved? And most importantly, how do you fix it? Don't worry, my friend. I've got your back. Let's dive into the problem, break it down, and find an easy solution.

Understanding the Error

To understand this error, we need to take a closer look at the code that triggered it. Here's a snippet of the code you provided:

interface Foo {
  name: string;
}

function go() {
  let instance: Foo | null = null;
  let mutator = () => {
    instance = {
      name: 'string'
    };
  };

  mutator();

  if (instance == null) {
    console.log('Instance is null or undefined');
  } else {
    console.log(instance.name);
  }
}

At first glance, everything seems fine. We have an interface called Foo with a property called name. Then, we have a function called go which initializes instance as null and defines a mutator function that modifies the instance by assigning it a new value.

However, when we try to access instance.name using console.log(instance.name), the error occurs: "Property 'name' does not exist on type 'never'."

Explaining the Error

To understand why this error is happening, we need to understand the concept of "type inference." In TypeScript, when a variable is assigned a value, TypeScript automatically infers its type based on the assigned value.

In the code you provided, instance is initially assigned a value of null, making TypeScript infer its type as null. Then, inside the mutator function, instance is assigned a new value of { name: 'string' }. At this point, TypeScript infers the type of instance to be { name: string }.

However, due to the conditional check if (instance == null), TypeScript realizes that instance could still be null. So it automatically adjusts its inferred type to never. The 'never' type represents values that will never occur. In this scenario, TypeScript considers it impossible for instance to hold the value { name: string } because it was previously assigned null.

Now, when we try to access instance.name, TypeScript analyzes the type mismatch and triggers the error message.

Resolving the Error

Luckily, resolving this error is quite straightforward. We have a couple of options:

1. Using Type Assertion

We can use type assertion to tell TypeScript that we know better than it does, and we guarantee that instance will not be null at the point of accessing instance.name. Here's how we can modify the code to include type assertion:

console.log((instance as Foo).name);

By asserting the type of instance as Foo, we let TypeScript know that it's safe to access the name property.

2. Refining the Type

Another way to resolve this error is by refining the type of instance using a type guard. This is useful when you want to conditionally handle different type variations. Here's an example:

function isNotNull<T>(value: T | null): value is T {
  return value !== null;
}

if (isNotNull(instance)) {
  console.log(instance.name);
} else {
  console.log('Instance is null or undefined');
}

In this code, we've created a type guard function called isNotNull that checks if a value is not null. By calling this function in an if statement, TypeScript can safely assume that if we reach the console.log(instance.name) line, instance is no longer null. Therefore, we can access its name property without triggering the error.

The Call-to-Action

Congratulations! 🎉 You've made it through the tricky world of "Property does not exist on type 'never'". Hopefully, this blog post helped you understand the error and provided you with easy solutions to resolve it.

If you found this blog post helpful, don't hesitate to share it with your fellow developers who might also be struggling with this error. And remember, there are always more exciting challenges waiting to be conquered in the tech world. Stay curious, keep learning, and happy coding! 💻😄