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TypeScript:UtilityTypes

lastmod 2024-01-27created 2023-03-0210 edits

TypeScript provides several utility types to facilitate common type transformations. These utilities are available globally.

Awaited<Type>

Partial<Type>

Constructs a type with all properties of Type set to optional. This utility returns a type that represents all subsets of a given type.

Required<Type>

Constructs a type consisting of all properties of Type set to required.

Readonly<Type>

Constructs a type with all properties of Type set to readonly, meaning the properties of the constructed type cannot be reassigned.

Record

Constructs a type with a set of properties Keys of type Type. This utility can be used to map the properties of a type to another type.

Pick

Constructs a type by picking the set of properties Keys from Type.

Omit

Constructs a type by picking all properties from Type and then removing Keys.

Exclude

Constructs a type by excluding from UnionType all properties that are assignable to ExcludedMembers.

Extract

Constructs a type by extracting from Type all properties that are assignable to Union.

NonNullable<Type>

Constructs a type by excluding null and undefined from Type.

Parameters<Type>

Constructs a tuple type from the types of the parameters of a function type Type.

ConstructorParameters<Type>

Lets you extract all parameter types of a constructor function type. It produces a tuple type with all the parameter types (or never if Type is not a function).

ReturnType<Type>

Constructs a type consisting of the return type of function Type.

InstanceType<Type>

Constructs a type consisting of the instance type of a constructor function type Type.

ThisParameterType<Type>

Extracts the type of the this parameter of a function type, or unknown if the function type has no this parameter.

Note: This type only works correctly when --strictFunctionTypes is enabled. See #32964.

OmitThisParameter<Type>

Removes the 'this' parameter from a function type.

Note: This type only works correctly when --strictFunctionTypes is enabled. See #32964.

ThisType<Type>

This utility does not return a transformed type. Instead, it serves as a marker for a contextual this type. Note that the --noImplicitThis flag must be used to use this utility.

Intrinsic String Manipulation Types

Uppercase<StringType>

Lowercase<StringType>

Capitalize<StringType>

Uncapitalize<StringType>

How to get the full list of keys from a specific instance 

export const appPaths = {
  login: '/login',
  loginPkceError: '/login/pkce/error',
  passwordResetUpdate: '/password/reset/update',
};

const propsArray = Object.keys(appPaths) as Array<keyof typeof appPaths>;
console.debug(propsArray);

How to turn a string array into string literals 

const arr = ["aaa", "bbb", "ccc"] as const;
type myCustomType = typeof arr[number];

and 

type myCustomType = "aaa" | "bbb" | "ccc";

are equivalent.

Changing interface property types based on a string literal 

interface PreferenceFieldCommon {
  key: string;
  label?: string;
  detail?: string;
}

type PreferenceFieldSpecialize =
  | {
      type: 'text';
    }
  | {
      type: 'select';
      options?: Array<SelectOptions>;
    }
  | {
      type: 'radio';
      options?: Array<RadioOptions>;
    };

export type PreferenceField = PreferenceFieldCommon & PreferenceFieldSpecialize;

Analysis

keyof keyword

The keyof keyword returns the keys of all properties present in a type as a union.

never type

never is a type that ignores errors without halting the process when they occur. It can be used as the primitive type of every other type except any. Although it is a type, you cannot actually declare a variable with the never type; it is mainly used as a function's return type, where it ignores errors when they occur and lets execution continue.

See also

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