Create a Union type from an Array or Object in TypeScript

# Table of Contents

1. Create a Union type from an Array in TypeScript
2. Create a Union type from an Object's Values or Keys in TS

# Create a Union type from an Array in TypeScript

To create a union type from an array:

1. Use as const to convert the array to a readonly tuple.
2. Use typeof arr[number] to get a type containing all of the array's elements.
3. The resulting type will be a union of all of the elements in the array.

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// 👇️ const sizes: readonly ["small", "medium", "large"]
const sizes = ['small', 'medium', 'large'] as const;

// 👇️ type SizesUnion = "small" | "medium" | "large"
type SizesUnion = typeof sizes[number];

We used as const to set the properties of the array to readonly and indicate to the language that the type of the expression will not be widened, e.g. from ['small', 'medium'] to string[].

The as const syntax is called a const assertion in TypeScript.

The array in the example becomes a readonly tuple, so its contents cannot be changed.

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// 👇️ const sizes: readonly ["small", "medium", "large"]
const sizes = ['small', 'medium', 'large'] as const;

// ⛔️ Error: Property 'push' does not exist
// on type 'readonly ["small", "medium", "large"]'
sizes.push('test');

If you try to change the contents of the array, you will get an error.

This way TypeScript can infer the type of the array as ['small', 'medium', 'large'] and not string[].

The typeof operator allows us to get the type of a variable.

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// 👇️ const sizes: readonly ["small", "medium", "large"]
const sizes = ['small', 'medium', 'large'] as const;

// 👇️ type SizesUnion = "small" | "medium" | "large"
type SizesUnion = typeof sizes[number];

// 👇️ type T = readonly ["small", "medium", "large"]
type T = typeof sizes;

Arrays have a numeric index signature.

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// 👇️ const sizes: readonly ["small", "medium", "large"]
const sizes = ['small', 'medium', 'large'] as const;

// 👇️ ['0', '1', '2']
console.log(Object.keys(sizes));

This is why we used the [number] syntax to get a union containing all of the values in the array.

We are basically getting all of the values in the array that can be indexed with a number key.

I've also written an article on how to get the type of the array elements from an array type.

# Create a Union type from an Object's Values or Keys in TS

If you need to create a union type from an object's values or keys:

1. Use as const to set the properties of the object to readonly.
2. Use keyof typeof to get a type of the keys in the object.
3. Use the keys to get a union of the values.

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// 👇️ const obj: {readonly name: "Bobby Hadz"; readonly country: "Chile";}
const obj = {
  name: 'Bobby Hadz',
  country: 'Chile',
} as const;

// 👇️ type UValues = "Bobby Hadz" | "Chile"
type UValues = (typeof obj)[keyof typeof obj];

// 👇️ type UKeys = "name" | "country"
type UKeys = keyof typeof obj;

We used as const to set the properties of the object to readonly and indicate to the language that the type of the expression will not be widened, e.g. from {name: "Bobby Hadz"} to {name: string}.

The as const syntax is called a const assertion in TypeScript.

The object in the example contains readonly properties, so their values cannot be changed.

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// 👇️ const obj: {readonly name: "Bobby Hadz"; readonly country: "Chile";}
const obj = {
  name: 'Bobby Hadz',
  country: 'Chile',
} as const;

// ⛔️ Error: Cannot assign to 'name' because it is a
// read-only property.
obj.name = 'James';

If you try to change the value of a property, you will get an error.

This way TypeScript can infer the type of the object as {name: 'Bobby hadz'; country: 'Chile'} instead of {name: string; country: string;}.

We used keyof typeof to create a union of the object's keys.

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// 👇️ const obj: {readonly name: "Bobby Hadz"; readonly country: "Chile";}
const obj = {
  name: 'Bobby Hadz',
  country: 'Chile',
} as const;

// 👇️ type UKeys = "name" | "country"
type UKeys = keyof typeof obj;

We can use the same approach to get a union of the object's values.

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// 👇️ const obj: {readonly name: "Bobby Hadz"; readonly country: "Chile";}
const obj = {
  name: 'Bobby Hadz',
  country: 'Chile',
} as const;

// 👇️ type UValues = "Bobby Hadz" | "Chile"
type UValues = typeof obj[keyof typeof obj];

// 👇️ type UKeys = "name" | "country"
type UKeys = keyof typeof obj;

We used the typeof operator to get the type of the object and then indexed the type with all of the keys to get a union containing the object's values.

I've also written a detailed article on how to create a type from an object's keys or values.