ts-essentials

All essential TypeScript types in one place 🤙

Install

npm install --save-dev ts-essentials

👉 We require typescript>=3.5. If you're looking for support for older TS versions use ts-essentials@2 instead.

What's inside?

• Install
• What's inside?
  • Basic
  • Dictionaries
  • Deep Partial & Deep Required & Deep Readonly & Deep NonNullable
  • Writable
  • Omit
  • StrictOmit
    • Comparison between Omit and StrictOmit
  • DeepOmit
  • OmitProperties
  • PickProperties
  • NonNever
  • Merge
  • MarkRequired
  • ReadonlyKeys
  • WritableKeys
  • UnionToIntersection
  • Opaque types
  • Tuple constraint
  • Literal types
  • Exhaustive switch cases
  • ValueOf type
  • AsyncOrSync type
• Contributors

Basic

• Primitive type matching all primitive values.

Dictionaries

const stringDict: Dictionary<string> = {
  a: "A",
  b: "B",
};

// Specify second type argument to change dictionary keys type
const dictOfNumbers: Dictionary<string, number> = {
  420: "four twenty",
  1337: "HAX",
};

// You may specify union types as key to cover all possible cases. It acts the same as Record from TS's standard library
export type DummyOptions = "open" | "closed" | "unknown";
const dictFromUnionType: Dictionary<number, DummyOptions> = {
  closed: 1,
  open: 2,
  unknown: 3,
};

// and get dictionary values
type stringDictValues = DictionaryValues<typeof stringDict>;
// Result: string

Deep Partial & Deep Required & Deep Readonly & Deep NonNullable

type ComplexObject = {
  simple: number;
  nested: {
    a: string;
    array: [{ bar: number }];
  };
};

type ComplexObjectPartial = DeepPartial<ComplexObject>;
const samplePartial: ComplexObjectPartial = {
  nested: {
    array: [{}],
  },
};

type ComplexObjectAgain = DeepRequired<ComplexObjectPartial>;
const sampleRequired: ComplexObjectAgain = {
  simple: 5,
  nested: {
    a: "test",
    array: [{bar: 1}],
  },
};

type ComplexObjectReadonly = DeepReadonly<ComplexObject>;

type ComplexNullableObject = {
  simple: number | null | undefined;
  nested: {
    a: string | null | undefined;
    array: [{ bar: number | null | undefined }] | null | undefined;
  };
};

type ComplexObjectNonNullable = DeepNonNullable<ComplexNullableObject>;
const sampleNonNullable: ComplexObjectNonNullable = {
  simple: 5,
  nested: {
    a: "test",
    array: [{bar: null}], // Error: Type 'null' is not assignable to type 'number'
  }
}

Writable

Make all attributes of object writable.

type Foo = {
  readonly a: number;
  readonly b: string;
};

const foo: Foo = ({ a: 1, b: "b" }(foo as Writable<typeof foo>).a = 42);

type Foo = {
  readonly foo: string;
  bar: {
    readonly x: number;
  };
}[];

const test: DeepWritable<Foo> = [
  {
    foo: "a",
    bar: {
      x: 5,
    },
  },
];

// we can freely write to this object
test[0].foo = "b";
test[0].bar.x = 2;

Omit

Our version of Omit is renamed to StrictOmit in v3, since the builtin Omit has become part of TypeScript 3.5

StrictOmit

Usage is similar to the builtin version, but checks the filter type more strictly.

type ComplexObject = {
  simple: number;
  nested: {
    a: string;
    array: [{ bar: number }];
  };
};

type SimplifiedComplexObject = StrictOmit<ComplexObject, "nested">;

// Result:
// {
//  simple: number
// }

// if you want to Omit multiple properties just use union type:
type SimplifiedComplexObject = StrictOmit<ComplexObject, "nested" | "simple">;

// Result:
// { } (empty type)

Comparison between Omit and StrictOmit

Following the code above, we can compare the behavior of Omit and StrictOmit.

type SimplifiedComplexObjectWithStrictOmit = StrictOmit<ComplexObject, "nested" | "simple" | "nonexistent">;

// Result: error
// Type '"simple" | "nested" | "nonexistent"' does not satisfy the constraint '"simple" | "nested"'.
// Type '"nonexistent"' is not assignable to type '"simple" | "nested"'.

type SimplifiedComplexObjectWithOmit = Omit<ComplexObject, "nested" | "simple" | "nonexistent">;

// Result: no error

As is shown in the example, StrictOmit ensures that no extra key is specified in the filter.

DeepOmit

Recursively omit deep properties according to key names.

Here is the Teacher interface.

interface Teacher {
  name: string,
  gender: string,
  students: {name: string, score: number}[]
}

Now suppose you want to omit gender property of Teacher, and score property of students. You can achieve this with a simple type filter.

In the filter, the properties to be omitted completely should be defined as never. For the properties you want to partially omit, you should recursively define the sub-properties to be omitted.

type TeacherSimple = DeepOmit<Teacher, {
  gender: never,
  students: {
    score: never,
  }
}>

// The result will be:
// {
//  name: string,
//  students: {name: string}[]
// }

NOTE

• DeepOmit works fine with Arrays and Sets. When applied to a Map, the filter is only applied to its value.
• If there exists any property in the filter which is not in the original type, an error will occur.

OmitProperties

Removes all properties extending type P in type T.

interface Example {
  log(): void;
  version: string;
}

type ExampleWithoutMethods = OmitProperties<Example, Function>;

// Result:
// {
//   version: string;
// }

// if you want to Omit multiple properties just use union type like

type ExampleWithoutMethods = OmitProperties<Example, Function | string>;
// Result:
// { } (empty type)

PickProperties

Pick only properties extending type P in type T.

interface Example {
  log(): void;
  version: string;
  versionNumber: number;
}

type ExampleOnlyMethods = PickProperties<Example, Function>;

// Result:
// {
//   log(): void;
// }

// if you want to pick multiple properties just use union type like

type ExampleOnlyMethodsAndString = PickProperties<Example, Function | string>;
// Result:
// {
//   log(): void;
//   version: string;
// }

NonNever

Useful for purifying object types. It improves intellisense but also allows for extracting keys satisfying a conditional type.

type GetDefined<TypesMap extends { [key: string]: any }> = keyof NonNever<
  { [T in keyof TypesMap]: TypesMap[T] extends undefined ? never : TypesMap[T] }
>;

NonEmptyObject

Useful for accepting only objects with keys, great after a filter like OmitProperties or PickProperties.

/* return never if the object doesn't have any number value*/
type NumberDictionary<T> = NonEmptyObject<PickProperties<T, number>>;

// return { a: number }
type SomeObject = NumberDictionary<{ a: number, b: string }>;

// return never
type EmptyObject = NumberDictionary<{}>;

Merge

type Foo = {
  a: number;
  b: string;
};

type Bar = {
  b: number;
};

const xyz: Merge<Foo, Bar> = { a: 4, b: 2 };
// Result:
// {
//   a: number,
//   b: number,
// }

MarkRequired

Useful when you're sure some optional properties will be set. A real life example: when selecting an object with its related entities from an ORM.

class User {
  id: number;
  posts?: Post[];
  photos?: Photo[];
}
type UserWithPosts = MarkRequired<User, 'posts'>;

// example usage with a TypeORM repository -- `posts` are now required, `photos` are still optional
async function getUserWithPosts(id: number): Promise<UserWithPosts> {
  return userRepo.findOneOrFail({ id }, { relations: ['posts'] }) as Promise<UserWithPosts>;
}

ReadonlyKeys

Gets keys of an object which are readonly.

type T = {
  readonly a: number;
  b: string;
};
type Result = ReadonlyKeys<T>
// Result:
// "a"

WritableKeys

Gets keys of an object which are writable.

type T = {
  readonly a: number;
  b: string;
};
type Result = WritableKeys<T>
// Result:
// "b"

UnionToIntersection

Useful for converting mapped types with function values to intersection type (so in this case - overloaded function).

type Foo = {
  bar: string;
  xyz: number;
};

type Fn = UnionToIntersection<{ [K in keyof Foo]: (type: K, arg: Foo[K]) => any }[keyof Foo]>;

Opaque types

type PositiveNumber = Opaque<number, "positive-number">;

function makePositiveNumber(n: number): PositiveNumber {
  if (n <= 0) {
    throw new Error("Value not positive !!!");
  }
  return (n as any) as PositiveNumber; // this ugly cast is required but only when "producing" opaque types
}

Tuple constraint

function foo<T extends Tuple>(tuple: T): T {
  return tuple;
}

const ret = foo(["s", 1]);
// return type of [string, number]

You can also parametrize Tuple type with a type argument to constraint it to certain types, i.e. Tuple<string | number>.

Literal types

For TypeScript >= 3.4: TypeScript 3.4 shipped const assertions which are very similar to our literal helper but also make type readonly, you should prefer as const construct. literal is deprecated tn ts-essentials 3.x, which requires TypeScript >=3.5.

For TypeScript < 3.4: this is served as a backport of the const assertions added since TypeScript 3.4.

// prevent type widening https://blog.mariusschulz.com/2017/02/04/typescript-2-1-literal-type-widening
const t = {
  letter: literal("a"), // type stays "a" not string
  digit: literal(5), // type stays 5 not number
};

Exhaustive switch cases

function actOnDummyOptions(options: DummyOptions): string {
  switch (options) {
    case "open":
      return "it's open!";
    case "closed":
      return "it's closed";
    case "unknown":
      return "i have no idea";
    default:
      // if you would add another option to DummyOptions, you'll get error here!
      throw new UnreachableCaseError(options);
  }
}

ValueOf type

const obj = {
  id: "123e4567-e89b-12d3-a456-426655440000",
  name: "Test object",
  timestamp: 1548768231486,
};

type objKeys = ValueOf<typeof obj>;
// Result: string | number

AsyncOrSync type

Useful as a return type in interfaces or abstract classes with missing implementation

interface CiProvider {
  getSHA(): AsyncOrSync<string>;
  // same as
  getSHA(): Promise<string> | string;
}

class Circle implements CiProvider {
  // implementation can use sync version
  getSHA() {
    return "abc";
  }
}

class Travis implements CiProvider {
  // implementation can use async version when needed
  async getSHA() {
    // do async call
    return "def";
  }
}

Contributors

Thanks goes to these wonderful people (emoji key):

Chris Kaczor 💻 💼 💡 📖	Xiao Liang 💻 🤔 📖	Mateusz Burzyński 💻 🤔 📖	Maciej Bembenista 💻 🤔 📖	Michael Tontchev 💻 🤔 📖	Thomas den Hollander 💻 🤔 📖	Esa-Matti Suuronen 💻 🤔 📖
Ilya Semenov 💻 🤔 📖	Code Checks 👀	Patricio Palladino 🤔	Artur Kozak 💻 🤔 📖 👀	Zihua Wu 💻 🤔 📖	Kevin Peno 💻	Dom Parfitt 🤔
EduardoRFS 💻 📖	Andrew C. Dvorak 📖

This project follows the all-contributors specification. Contributions of any kind welcome! Read more