Chapter 7

Mutability

7

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In our chapter on unions and narrowing, we explored how TypeScript can infer types from the logical flow of our code. In this chapter, we'll see how mutability - whether a value can be changed or not - can affect type inference.

Mutability and Inference

Variable Declaration and Type Inference

How you declare your variables in TypeScript affects whether or not they can be changed.

How TypeScript Infers let

When using the let keyword, the variable is mutable and can be reassigned.

Consider this AlbumGenre type: a union of literal values representing possible genres for an album:

type AlbumGenre = "rock" | "country" | "electronic";

Using let, we can declare a variable albumGenre and assign it the value "rock". Then we can attempt to pass albumGenre to a function that expects an AlbumGenre:

let albumGenre = "rock";

const handleGenre = (genre: AlbumGenre) => {
  // ...
};

handleGenre(albumGenre);
Argument of type 'string' is not assignable to parameter of type 'AlbumGenre'.2345
Argument of type 'string' is not assignable to parameter of type 'AlbumGenre'.

Because let was used when declaring the variable, TypeScript understands that the value can later be changed. In this case, it infers albumGenre as a string rather than the specific literal type "rock". In our code, we could do this:

albumGenre = "country";

Therefore, it will infer a wider type in order to accommodate the variable being reassigned.

We can fix the error above by assigning a specific type to the let:

let albumGenre: AlbumGenre = "rock";

const handleGenre = (genre: AlbumGenre) => {
  // ...
};

handleGenre(albumGenre); // no more error

Now, albumGenre can be reassigned, but only to a value that is a member of the AlbumGenre union. So, it will no longer show an error when passed to handleGenre.

But there's another interesting solution.

How TypeScript Infers const

When using const, the variable is immutable and cannot be reassigned. When we change the variable declaration to use const, TypeScript will infer the type more narrowly:

const albumGenre = "rock";

const handleGenre = (genre: AlbumGenre) => {
  // ...
};

handleGenre(albumGenre); // No error

There is no longer an error in the assignment, and hovering over albumGenre inside of the albumDetails object shows that TypeScript has inferred it as the literal type "rock".

If we try to change the value of albumGenre after declaring it as const, TypeScript will show an error:

albumGenre = "country";
Cannot assign to 'albumGenre' because it is a constant.2588
Cannot assign to 'albumGenre' because it is a constant.

TypeScript is mirroring JavaScript's treatment of const in order to prevent possible runtime errors. When you declare a variable with const, TypeScript infers it as the literal type you specified.

So, TypeScript uses how JavaScript works to its advantage. This will often encourage you to use const over let when declaring variables, as it's a little stricter.

Object Property Inference

The picture with const and let becomes a bit more complicated when it comes to object properties.

Objects are mutable in JavaScript, meaning their properties can be changed after they are created.

For this example, we have an AlbumAttributes type that includes a status property with a union of literal values representing possible album statuses:

type AlbumAttributes = {
  status: "new-release" | "on-sale" | "staff-pick";
};

Say we had an updateStatus function that takes an AlbumAttributes object:

const updateStatus = (attributes: AlbumAttributes) => {
  // ...
};

const albumAttributes = {
  status: "on-sale",
};

updateStatus(albumAttributes);
Argument of type '{ status: string; }' is not assignable to parameter of type 'AlbumAttributes'. Types of property 'status' are incompatible. Type 'string' is not assignable to type '"new-release" | "on-sale" | "staff-pick"'.2345
Argument of type '{ status: string; }' is not assignable to parameter of type 'AlbumAttributes'. Types of property 'status' are incompatible. Type 'string' is not assignable to type '"new-release" | "on-sale" | "staff-pick"'.

TypeScript gives us an error below albumAttributes inside of the updateStatus function call, with messages similar to what we saw before.

This is happening because TypeScript has inferred the status property as a string rather than the specific literal type "on-sale". Similar to with let, TypeScript understands that the property could later be reassigned:

albumAttributes.status = "new-release";

This is true even though the albumAttributes object was declared with const. We get the error when calling updateStatus because status: string can't be passed to a function that expects status: "new-release" | "on-sale" | "staff-pick". TypeScript is trying to protect us from potential runtime errors.

Let's look at a couple of ways to fix this issue.

Using an Inline Object

One approach is to inline the object when calling the updateStatus function instead of declaring it separately:

updateStatus({
  status: "on-sale",
}); // No error

When inlining the object, TypeScript knows that there is no way that status could be changed before it is passed into the function, so it infers it more narrowly.

Adding a Type to the Object

Another option is to explicitly declare the type of the albumAttributes object to be AlbumAttributes:

const albumAttributes: AlbumAttributes = {
  status: "on-sale",
};

updateStatus(albumAttributes); // No error

This works similarly to how it did with the let. While albumAttributes.status can still be reassigned, it can only be reassigned to a valid value:

albumAttributes.status = "new-release"; // No error

This behaviour works the same for all object-like structures, including arrays and tuples. We'll examine those later in the exercises.

Readonly Object Properties

In JavaScript, as we've seen, object properties are mutable by default. But TypeScript lets us be more specific about whether or not a property of an object can be mutated.

To make a property read-only (not writable), you can use the readonly modifier:

Consider this Album interface, where the title and artist are marked as readonly:

interface Album {
  readonly title: string;
  readonly artist: string;
  status?: "new-release" | "on-sale" | "staff-pick";
  genre?: string[];
}

Once an Album object is created, its title and artist properties are locked in and cannot be changed. However, the optional status and genre properties can still be modified.

Note that this only occurs on the type level. At runtime, the properties are still mutable. TypeScript is just helping us catch potential errors.

The Readonly Type Helper

If you want to specify that all properties of an object should be read-only, TypeScript provides a type helper called Readonly.

To use it, you simply wrap the object type with Readonly.

Here's an example of using Readonly to create an Album object:

const readOnlyWhiteAlbum: Readonly<Album> = {
  title: "The Beatles (White Album)",
  artist: "The Beatles",
  status: "staff-pick",
};

Because the readOnlyWhiteAlbum object was created using the Readonly type helper, none of the properties can be modified:

readOnlyWhiteAlbum.genre = ["rock", "pop", "unclassifiable"];
Cannot assign to 'genre' because it is a read-only property.2540
Cannot assign to 'genre' because it is a read-only property.

Note that like many of TypeScript's type helpers, the immutability enforced by Readonly only operates on the first level. It won't make properties read-only recursively.

Readonly Arrays

As with object properties, arrays and tuples can also be made immutable by using the readonly modifier.

Here's how the readonly modifier can be used to create a read-only array of genres. Once the array is created, its contents cannot be modified:

const readOnlyGenres: readonly string[] = ["rock", "pop", "unclassifiable"];

Similar to the Array syntax, TypeScript also offers a ReadonlyArray type helper that functions in the same way to using the above syntax:

const readOnlyGenres: ReadonlyArray<string> = ["rock", "pop", "unclassifiable"];

Both of these approaches are functionally the same. Hovering over the readOnlyGenres variable shows that TypeScript has inferred it as a read-only array:

// hovering over `readOnlyGenres` shows:
const readOnlyGenres: readonly string[];

Readonly arrays disallow the use of array methods that cause mutations, such as push and pop:

readOnlyGenres.push("experimental");
Property 'push' does not exist on type 'readonly string[]'.2339
Property 'push' does not exist on type 'readonly string[]'.

However, methods like map and reduce will still work, as they create a copy of the array and do not mutate the original.

const uppercaseGenres = readOnlyGenres.map((genre) => genre.toUpperCase()); // No error

readOnlyGenres.push("experimental");
Property 'push' does not exist on type 'readonly string[]'.2339
Property 'push' does not exist on type 'readonly string[]'.

Note that, just like the readonly for object properties, this doesn't affect the runtime behavior of the array. It's just a way to help catch potential errors.

How Read-Only and Mutable Arrays Work Together

To help drive the concept home, let's see how read-only and mutable arrays work together.

Here are two printGenre functions that are functionally identical, except printGenresReadOnly takes a read-only array of genres as a parameter whereas printGenresMutable takes a mutable array:

function printGenresReadOnly(genres: readonly string[]) {
  // ...
}

function printGenresMutable(genres: string[]) {
  // ...
}

When we create a mutable array of genres, it can be passed as an argument to both of these functions without error:

const mutableGenres = ["rock", "pop", "unclassifiable"];

printGenresReadOnly(mutableGenres);
printGenresMutable(mutableGenres);

This works because specifying readonly on the printGenresReadOnly function parameter only guarantees that it won't alter the array's content. Thus, it doesn't matter if we pass a mutable array because it won't be changed.

However, the reverse is not true.

If we declare a read-only array, we can only pass it to printGenresReadOnly. Attempting to pass it to printGenresMutable will yield an error:

const readOnlyGenres: readonly string[] = ["rock", "pop", "unclassifiable"];

printGenresReadOnly(readOnlyGenres);
printGenresMutable(readOnlyGenres);
Argument of type 'readonly string[]' is not assignable to parameter of type 'string[]'. The type 'readonly string[]' is 'readonly' and cannot be assigned to the mutable type 'string[]'.2345
Argument of type 'readonly string[]' is not assignable to parameter of type 'string[]'. The type 'readonly string[]' is 'readonly' and cannot be assigned to the mutable type 'string[]'.

This is because we might be mutating the array inside of printGenresMutable. If we passed a read-only array.

Essentially, read-only arrays can only be assigned to other read-only types. This can spread virally throughout your application: if a function deep down the call stack expects a readonly array, then that array must remain readonly throughout. But doing so brings benefits. It ensures that the array won't be mutated in any manner as it moves down the stack. Very useful.

The takeaway here is that even though you can assign mutable arrays to read-only arrays, you cannot assign read-only arrays to mutable arrays.

Exercises

Exercise 1: Inference with an Array of Objects

Here we have a modifyButtons function that takes in an array of objects with type properties that are either "button", "submit", or "reset".

When attempting to call modifyButtons with an array of objects that seem to meet the contract, TypeScript gives us an error:

type ButtonAttributes = {
  type: "button" | "submit" | "reset";
};

const modifyButtons = (attributes: ButtonAttributes[]) => {};

const buttonsToChange = [
  {
    type: "button",
  },
  {
    type: "submit",
  },
];

modifyButtons(buttonsToChange);
Argument of type '{ type: string; }[]' is not assignable to parameter of type 'ButtonAttributes[]'. Type '{ type: string; }' is not assignable to type 'ButtonAttributes'. Types of property 'type' are incompatible. Type 'string' is not assignable to type '"button" | "submit" | "reset"'.2345
Argument of type '{ type: string; }[]' is not assignable to parameter of type 'ButtonAttributes[]'. Type '{ type: string; }' is not assignable to type 'ButtonAttributes'. Types of property 'type' are incompatible. Type 'string' is not assignable to type '"button" | "submit" | "reset"'.

Your task is to determine why this error shows up, then resolve it.

Exercise 1: Inference with an Array of Objects

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Exercise 2: Avoiding Array Mutation

This printNames function accepts an array of name strings and logs them to the console. However, there are also non-working @ts-expect-error comments that should not allow for names to be added or changed:

function printNames(names: string[]) {
  for (const name of names) {
    console.log(name);
  }

  // @ts-expect-error
Unused '@ts-expect-error' directive.2578
Unused '@ts-expect-error' directive. names.push("John"); // @ts-expect-error
Unused '@ts-expect-error' directive.2578
Unused '@ts-expect-error' directive. names[0] = "Billy"; }

Your task is to update the type of the names parameter so that the array cannot be mutated. There are two ways to solve this problem.

Exercise 2: Avoiding Array Mutation

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Exercise 3: An Unsafe Tuple

Here we have a dangerousFunction which accepts an array of numbers as an argument:

const dangerousFunction = (arrayOfNumbers: number[]) => {
  arrayOfNumbers.pop();
  arrayOfNumbers.pop();
};

Additionally, we've defined a variable myHouse which is a tuple representing a Coordinate:

type Coordinate = [number, number];
const myHouse: Coordinate = [0, 0];

Our tuple myHouse contains two elements, and the dangerousFunction is structured to pop two elements from the given array.

Given that pop removes the last element from an array, calling dangerousFunction with myHouse will remove its contents.

Currently, TypeScript does not alert us to this potential issue, as seen by the error line under @ts-expect-error:

dangerousFunction(
  // @ts-expect-error
Unused '@ts-expect-error' directive.2578
Unused '@ts-expect-error' directive. myHouse, );

Your task is to adjust the type of Coordinate such that TypeScript triggers an error when we attempt to pass myHouse into dangerousFunction.

Note that you should only change Coordinate, and leave the function untouched.

Exercise 3: An Unsafe Tuple

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Solution 1: Inference with an Array of Objects

Hovering over the buttonsToChange variable shows us that it is being inferred as an array of objects with a type property of type string:

// hovering over buttonsToChange shows:
const buttonsToChange: {
  type: string;
}[];

This inference is happening because our array is mutable. We could change the type of the first element in the array to something different:

buttonsToChange[0].type = "something strange";

This wider type is incompatible with the ButtonAttributes type, which expects the type property to be one of "button", "submit", or "reset".

The fix here is to specify that buttonsToChange is an array of ButtonAttributes:

type ButtonAttributes = {
  type: "button" | "submit" | "reset";
};

const modifyButton = (attributes: ButtonAttributes[]) => {};

const buttonsToChange: ButtonAttributes[] = [
  {
    type: "button",
  },
  {
    type: "submit",
  },
];

modifyButtons(buttonsToChange); // No error

Or, we could pass the array directly to the modifyButtons function:

modifyButtons([
  {
    type: "button",
  },
  {
    type: "submit",
  },
]); // No error

By doing this, TypeScript will infer the type property more narrowly, and the error will go away.

Solution 2: Avoiding Array Mutation

Here are a couple ways to solve this problem.

Option 1: Add the readonly Keyword

The first approach solution is to add the readonly keyword before the string[] array. It applies to the entire string[] array, converting it into a read-only array:

function printNames(names: readonly string[]) {
  ...
}

With this setup, TypeScript won't allow you to add items with .push() or perform any other modifications on the array.

Option 2: Use the ReadonlyArray Type Helper

Alternatively, you could use the ReadonlyArray type helper:

function printNames(names: ReadonlyArray<string>) {
  ...
}

Regardless of which of these two methods you use, TypeScript will still display readonly string[] when hovering over the names parameter:

// hovering over `names` shows:
(parameter) names: readonly string[]

Both work equally well at preventing the array from being modified.

Solution 3: An Unsafe Tuple

The best way to prevent unwanted changes to the Coordinate tuple is to make it a readonly tuple:

type Coordinate = readonly [number, number];

Now, dangerousFunction throws a TypeScript error when we try to pass myHouse to it:

const dangerousFunction = (arrayOfNumbers: number[]) => {
  arrayOfNumbers.pop();
  arrayOfNumbers.pop();
};

dangerousFunction(myHouse);
Argument of type 'Coordinate' is not assignable to parameter of type 'number[]'. The type 'Coordinate' is 'readonly' and cannot be assigned to the mutable type 'number[]'.2345
Argument of type 'Coordinate' is not assignable to parameter of type 'number[]'. The type 'Coordinate' is 'readonly' and cannot be assigned to the mutable type 'number[]'.

We get an error because the function's signature expects a modifiable array of numbers, but myHouse is a read-only tuple. TypeScript is protecting us against unwanted changes.

It's a good practice to use readonly tuples as much as possible to avoid problems like the one in this exercise.

Deep Immutability with as const

We've seen so far that objects and arrays are mutable in JavaScript. This leads to their properties being inferred widely by TypeScript.

We can get around this by giving the property a type annotation. But it still doesn't infer the literal type of the property.

const albumAttributes: AlbumAttributes = {
  status: "on-sale",
};

// hovering over albumAttributes shows:
const albumAttributes: {
  status: "new-release" | "on-sale" | "staff-pick";
};

Instead of albumAttributes.status being inferred as "on-sale", it's inferred as "new-release" | "on-sale" | "staff-pick".

One way we could get TypeScript to infer it properly would be to somehow mark the entire object, and all its properties, as immutable. This would tell TypeScript that the object and its properties can't be changed, so it would be free to infer the literal types of the properties.

This is where the as const assertion comes in. We can use it to mark an object and all of its properties as constants, meaning that they can't be changed once they are created.

const albumAttributes = {
  status: "on-sale",
} as const;

// hovering over albumAttributes shows:
const albumAttributes: {
  readonly status: "on-sale";
};

The as const assertion has made the entire object deeply read-only, including all of its properties. This means that albumAttributes.status is now inferred as the literal type "on-sale".

Attempting to change the status property will result in an error:

albumAttributes.status = "new-release";
Cannot assign to 'status' because it is a read-only property.2540
Cannot assign to 'status' because it is a read-only property.

This makes as const ideal for large config objects that you don't expect to change.

Just like the readonly modifier, as const only affects the type level. At runtime, the object and its properties are still mutable.

as const vs Variable Annotation

You might be wondering what would happen if we combined as const with a variable annotation. How would it be inferred?

const albumAttributes: AlbumAttributes = {
  status: "on-sale",
} as const;

You can think of this code as a competition between two forces: the as const assertion operating on the value, and the annotation operating on the variable.

When you have a competition like this, the variable annotation wins. The variable owns the value, and forgets whatever the explicit value was before.

This means, curiously, that the status property is inferred as being mutable:

albumAttributes.status = "new-release"; // No error

The as const assertion is being overridden by the variable annotation. Not fun.

We'll explore this interaction between variables and values further in our chapter on annotations and assertions.

Comparing as const with Object.freeze

In JavaScript, the Object.freeze method is a way to create immutable objects at runtime. There are some significant differences between Object.freeze and as const.

For this example, we'll create a shelfLocations object that uses Object.freeze:

const shelfLocations = Object.freeze({
  entrance: {
    status: "on-sale",
  },
  frontCounter: {
    status: "staff-pick",
  },
  endCap: {
    status: "new-release",
  },
});

Hovering over shelfLocations shows that the object has the Readonly modifier applied to it:

// hovering over shelfLocations shows:
const shelfLocations: Readonly<{
  entrance: {
    status: string;
  };
  frontCounter: {
    status: string;
  };
  endCap: {
    status: string;
  };
}>;

Recall that the Readonly modifier only works on the first level of an object. If we try to modify the frontCounter property, TypeScript will throw an error:

shelfLocations.frontCounter = {
Cannot assign to 'frontCounter' because it is a read-only property.2540
Cannot assign to 'frontCounter' because it is a read-only property. status: "new-release", };

However, we are able to change the nested status property of a specific location:

shelfLocations.entrance.status = "new-release";

This is in line with how Object.freeze works in JavaScript. It only makes the object and its properties read-only at the first level. It doesn't make the entire object deeply read-only.

Using as const makes the entire object deeply read-only, including all nested properties:

const shelfLocations = {
  entrance: {
    status: "on-sale",
  },
  frontCounter: {
    status: "staff-pick",
  },
  endCap: {
    status: "new-release",
  },
} as const;

console.log(shelfLocations);
const shelfLocations: { readonly entrance: { readonly status: "on-sale"; }; readonly frontCounter: { readonly status: "staff-pick"; }; readonly endCap: { readonly status: "new-release"; }; }

Of course, this is just a type-level annotation. Object.freeze gives you runtime immutability, while as const gives you type-level immutability. I actually prefer the latter - doing less work at runtime is always a good thing.

So while both as const and Object.freeze will enforce immutability, as const is the more convenient and efficient choice. Unless you specifically need the top level of an object to be frozen at runtime, you should stick with as const.

Exercises

Exercise 1: Returning A Tuple From A Function

In this exercise, we are dealing with an async function named fetchData that fetches data from a URL and returns a result.

Whether the function succeeds or fails, it returns a tuple. The first member of the tuple contains an error message if the fetch operation fails, and the second member contains the fetched data if the operation is successful.

Here's how the function is currently implemented:

const fetchData = async () => {
  const result = await fetch("/");

  if (!result.ok) {
    return [new Error("Could not fetch data.")];
  }

  const data = await result.json();

  return [undefined, data];
};

Here's an async example function that uses fetchData and includes a couple of test cases:

const example = async () => {
  const [error, data] = await fetchData();

  type Tests = [
    Expect<Equal<typeof error, Error | undefined>>,
Type 'false' does not satisfy the constraint 'true'.2344
Type 'false' does not satisfy the constraint 'true'. Expect<Equal<typeof data, any>>, ]; };

Currently, both members of the tuple are inferred as any, which isn't ideal.

const [error, data] = await fetchData();

// hovering over error and data shows:
const error: any;
const data: any;

Your challenge is to modify the fetchData function implementation so that TypeScript infers a Promise with a tuple for its return type.

Depending on whether or not the fetch operation is successful, the tuple should contain either an error message or a pair of undefined and the data fetched.

Hint: There are two possible approaches to solve this challenge. One way would be to define an explicit return type for the function. Alternatively, you could attempt to add or change type annotations for the return values within the function.

Exercise 1: Returning A Tuple From A Function

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Exercise 2: Inferring Literal Values In Arrays

Let's revisit a previous exercise and evolve our solution.

The modifyButtons function accepts an array of objects with a type property:

type ButtonAttributes = {
  type: "button" | "submit" | "reset";
};

const modifyButtons = (attributes: ButtonAttributes[]) => {};

const buttonsToChange = [
  {
    type: "button",
  },
  {
    type: "submit",
  },
];

modifyButtons(buttonsToChange);
Argument of type '{ type: string; }[]' is not assignable to parameter of type 'ButtonAttributes[]'. Type '{ type: string; }' is not assignable to type 'ButtonAttributes'. Types of property 'type' are incompatible. Type 'string' is not assignable to type '"button" | "submit" | "reset"'.2345
Argument of type '{ type: string; }[]' is not assignable to parameter of type 'ButtonAttributes[]'. Type '{ type: string; }' is not assignable to type 'ButtonAttributes'. Types of property 'type' are incompatible. Type 'string' is not assignable to type '"button" | "submit" | "reset"'.

Previously, the error was solved by updating buttonsToChange to be specified as an array of ButtonAttributes:

const buttonsToChange: ButtonAttributes[] = [
  {
    type: "button",
  },
  {
    type: "submit",
  },
];

This time, your challenge is to solve the error by finding a different solution. Specifically, you should modify the buttonsToChange array so that TypeScript infers the literal type of the type property.

You should not alter the ButtonAttributes type definition or the modifyButtons function.

Exercise 2: Inferring Literal Values In Arrays

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Solution 1: Returning A Tuple From A Function

As mentioned, there are two different solutions to this challenge.

Option 1: Defining a Return Type

The first solution is to define a return type for the fetchData function.

Inside the Promise type, a tuple is defined with either Error or undefined as the first member, and an optional any as the second member:

const fetchData = async (): Promise<[Error | undefined, any?]> => {
  ...

This technique works perfectly well.

Option 2: Using as const

Instead of specifying a return type, the second solution is to use as const on the return values:

import { Equal, Expect } from "@total-typescript/helpers";

const fetchData = async () => {
  const result = await fetch("/");

  if (!result.ok) {
    return [new Error("Could not fetch data.")] as const; // added as const here
  }

  const data = await result.json();

  return [undefined, data] as const; // added as const here
};

With these changes in place, when we check the return type of fetchData in the example function, we can see that error is inferred as Error | undefined, and data is any:

const example = async () => {
  const [error, data] = await fetchData();

  // ...
};

// hovering over error shows:
const error: Error | undefined;

// hovering over data shows:
const data: any;

In the case of this challenge, without as const, TypeScript is making two mistakes. Firstly, it's inferring each of the returned arrays as arrays, not tuples. This is TypeScript's default behaviour:

const data = await result.json();

const result = [undefined, data];

// hovering over result shows:
const result: any[];

We can also see that when undefined is placed into an array with any, TypeScript infers the array as any[]. This is TypeScript's second mistake - collapsing our undefined value so it all but disappears.

However, by using as const, TypeScript correctly infers the return value as a tuple (Promise<[string | undefined, any]>). This is a great example of how as const can help TypeScript give us the best type inference possible.

Solution 2: Inferring Literal Values In Arrays

Let's look at some different options for solving this challenge.

Option 1: Annotate the Entire Array

The as const assertion can be used to solve this problem. By annotating the entire array with as const, TypeScript will infer the literal type of the type property:

const buttonsToChange = [
  {
    type: "button",
  },
  {
    type: "submit",
  },
] as const;

Hovering over buttonsToChange shows that TypeScript has inferred the type property as a literal type, and modifyButtons will no longer show an error when buttonsToChange is passed to it:

// hovering over buttonsToChange shows:
const buttonsToChange: readonly [
  {
    readonly type: "button";
  },
  {
    readonly type: "submit";
  },
];
Option 2: Annotate the members of the array

Another way to solve this problem is to annotate each member of the array with as const:

const buttonsToChange = [
  {
    type: "button",
  } as const,
  {
    type: "submit",
  } as const,
];

Hovering over buttonsToChange shows something interesting. Each object is now inferred as readonly, but the array itself is not:

// hovering over buttonsToChange shows:
const buttonsToChange: (
  | {
      readonly type: "button";
    }
  | {
      readonly type: "submit";
    }
)[];

The buttonsToChange array is also no longer being inferred as a tuple with a fixed length, so we can modify it by pushing new objects to it:

buttonsToChange.push({
  type: "button",
});

This behavior stems from tagging the individual members of the array with as const, instead of the entire array.

However, this inference is good enough to satisfy modifyButtons, because it matches the ButtonAttributes type.

Option 3: as const on strings

The last solution we'll look at is using as const on the string literals to infer the literal type:

const buttonsToChange = [
  {
    type: "button" as const,
  },
  {
    type: "submit" as const,
  },
];

Now when we hover over buttonsToChange we've lost the readonly modifier, because as const is only being targeted at the string inside of the object, not the object itself:

// hovering over buttonsToChange shows:
const buttonsToChange: (
  | {
      type: "button";
    }
  | {
      type: "submit";
    }
)[];

But again, this is still typed strongly enough to to satisfy modifyButtons.

When using as const like this acts like a hint to TypeScript that it should infer a literal type where it wouldn't otherwise. This can be occasionally useful for when you want to allow mutation, but still want to infer a literal type.

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