How Do You Cast A Potentially Readonly Key To Readwrite Key In TypeScript?

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Introduction

In TypeScript, when working with objects, we often need to access their properties using keys. However, when these keys are derived from a keyof type, they can be either readonly or readwrite. In this article, we will explore how to cast a potentially readonly key to a readwrite key in TypeScript.

Understanding Key Types in TypeScript

In TypeScript, a keyof type is a type that represents the keys of an object. When we use the keyof operator on an object type, it returns a type that represents the keys of that object. For example, if we have an object type Foo, the keyof Foo type would represent the keys of Foo.

class Foo {
  x = 1
  readonly y = 2
}

type FooKeys = keyof Foo
// type FooKeys = 'x' | 'y'

In the above example, FooKeys is a type that represents the keys of Foo. However, since y is a readonly property, the FooKeys type does not include it.

Casting a Potentially Readonly Key to Readwrite

When we need to access a property using a key that is derived from a keyof type, we may encounter issues if the key is readonly. To overcome this, we can use the as keyword to cast the key to a readwrite key.

class Foo {
  x = 1
  readonly y = 2
}

const foo = new Foo()


const k = 'x' as keyof Foo &  [P in keyof Foo] never 
// type k = 'x'


foo[k] = 3
// foo.x = 3

In the above example, we use the as keyword to cast the k variable to a type that includes both the readonly and readwrite keys of Foo. The & { [P in keyof Foo]: never } part is used to create a type that includes all the keys of Foo, including the readonly ones.

Using the as Keyword with keyof and never

When we use the as keyword with keyof and never, we can create a type that includes all the keys of an object, including the readonly ones. This is because the never type is a type that represents a value that is never reachable. By using never in the type, we can create a type that includes all the keys of an object.

class Foo {
  x = 1
  readonly y = 2
}

type FooKeys = keyof Foo &  [P in keyof Foo] never 
// type FooKeys = 'x' | 'y'

In the above example, FooKeys is a type that includes both the readonly and readwrite keys of Foo.

Using the as Keyword with keyof and Readonly

When we use the as keyword with keyof and Readonly, we can create a type that includes all the keys of an object, including the readonly ones. This is because the Readonly type is a type that represents a readonly value.

class Foo {
  x = 1
  readonly y = 2
}

type FooKeys = keyof Foo & Readonly<keyof Foo>
// type FooKeys = 'x' | 'y'

In the above example, FooKeys is a type that includes both the readonly and readwrite keys of Foo.

Conclusion

In this article, we have explored how to cast a potentially readonly key to a readwrite key in TypeScript. We have seen how to use the as keyword with keyof and never to create a type that includes all the keys of an object, including the readonly ones. We have also seen how to use the as keyword with keyof and Readonly to create a type that includes all the keys of an object, including the readonly ones.

Example Use Cases

Here are some example use cases for casting a potentially readonly key to a readwrite key in TypeScript:

  • Accessing a readonly property: When we need to access a readonly property using a key that is derived from a keyof type, we can use the as keyword to cast the key to a readwrite key.
  • Updating a readonly property: When we need to update a readonly property using a key that is derived from a keyof type, we can use the as keyword to cast the key to a readwrite key.
  • Creating a type that includes all keys: When we need to create a type that includes all the keys of an object, including the readonly ones, we can use the as keyword with keyof and never or Readonly.

Playground

Here is a playground example that demonstrates how to cast a potentially readonly key to a readwrite key in TypeScript:

class Foo {
  x = 1
  readonly y = 2
}

const foo = new Foo()


const k = 'x' as keyof Foo &  [P in keyof Foo] never 
// type k = 'x'


foo[k] = 3
// foo.x = 3


console.log(foo.x) // 3
console.log(foo.y) // 2

In this example, we use the as keyword to cast the k variable to a type that includes both the readonly and readwrite keys of Foo. We then use the k variable to access and update the properties of foo.

Introduction

In our previous article, we explored how to cast a potentially readonly key to a readwrite key in TypeScript. In this article, we will answer some frequently asked questions about casting a potentially readonly key to a readwrite key in TypeScript.

Q1: What is the difference between a readonly key and a readwrite key?

A1: A readonly key is a key that cannot be used to update a property, while a readwrite key is a key that can be used to update a property.

Q2: Why do I need to cast a potentially readonly key to a readwrite key?

A2: You need to cast a potentially readonly key to a readwrite key when you need to access or update a property using a key that is derived from a keyof type, but the property is readonly.

Q3: How do I cast a potentially readonly key to a readwrite key?

A3: You can cast a potentially readonly key to a readwrite key using the as keyword with keyof and never or Readonly.

Q4: What is the never type in TypeScript?

A4: The never type in TypeScript is a type that represents a value that is never reachable. It is used to create a type that includes all the keys of an object, including the readonly ones.

Q5: What is the Readonly type in TypeScript?

A5: The Readonly type in TypeScript is a type that represents a readonly value. It is used to create a type that includes all the keys of an object, including the readonly ones.

Q6: Can I use the as keyword with keyof and never or Readonly in a type guard?

A6: Yes, you can use the as keyword with keyof and never or Readonly in a type guard. However, you need to be careful when using type guards, as they can affect the type of the variable.

Q7: What are some common use cases for casting a potentially readonly key to a readwrite key?

A7: Some common use cases for casting a potentially readonly key to a readwrite key include accessing a readonly property, updating a readonly property, and creating a type that includes all keys.

Q8: Can I use the as keyword with keyof and never or Readonly in a function parameter?

A8: Yes, you can use the as keyword with keyof and never or Readonly in a function parameter. However, you need to be careful when using function parameters, as they can affect the type of the function.

Q9: What are some best practices for using the as keyword with keyof and never or Readonly?

A9: Some best practices for using the as keyword with keyof and never or Readonly include using it sparingly, being careful with type guards, and documenting the usage of the as keyword.

Q10: Can I use the as keyword with keyof and never or Readonly in a class property?

A10: Yes, you can use the as keyword with keyof and never or Readonly in a class property. However, you need to be careful when using class properties, as they can affect the type of the class.

Conclusion

In this article, we have answered some frequently asked questions about casting a potentially readonly key to a readwrite key in TypeScript. We have also provided some best practices for using the as keyword with keyof and never or Readonly.

Example Use Cases

Here are some example use cases for casting a potentially readonly key to a readwrite key in TypeScript:

  • Accessing a readonly property: When we need to access a readonly property using a key that is derived from a keyof type, we can use the as keyword to cast the key to a readwrite key.
  • Updating a readonly property: When we need to update a readonly property using a key that is derived from a keyof type, we can use the as keyword to cast the key to a readwrite key.
  • Creating a type that includes all keys: When we need to create a type that includes all the keys of an object, including the readonly ones, we can use the as keyword with keyof and never or Readonly.

Playground

Here is a playground example that demonstrates how to cast a potentially readonly key to a readwrite key in TypeScript:

class Foo {
  x = 1
  readonly y = 2
}

const foo = new Foo()


const k = 'x' as keyof Foo &  [P in keyof Foo] never 
// type k = 'x'


foo[k] = 3
// foo.x = 3


console.log(foo.x) // 3
console.log(foo.y) // 2

In this example, we use the as keyword to cast the k variable to a type that includes both the readonly and readwrite keys of Foo. We then use the k variable to access and update the properties of foo.