Marius Schulz
Marius Schulz
Front End Engineer

The Omit Helper Type in TypeScript

In version 3.5, TypeScript added an Omit<T, K> helper type to the lib.es5.d.ts type definition file that ships as part of the TypeScript compiler. The Omit<T, K> type lets us create an object type that omits specific properties from another object type:

type User = {
  id: string;
  name: string;
  email: string;
};

type UserWithoutEmail = Omit<User, "email">;

// This is equivalent to:
type UserWithoutEmail = {
  id: string;
  name: string;
};

The Omit<T, K> helper type is defined in lib.es5.d.ts like this:

/**
 * Construct a type with the properties of T except for those in type K.
 */
type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>;

To untangle this type definition and understand how it works, let's see how we could've come up with our own version of the Omit<T, K> helper type ourselves.

Defining the Omit<T, K> Helper Type

Let's start with the same User type we've seen above:

type User = {
  id: string;
  name: string;
  email: string;
};

First, we need to be able to retrieve all keys of the User type. We can use the keyof operator to retrieve a union of string literal types that contains all property keys of this object type:

type UserKeys = keyof User;

// This is equivalent to:
type UserKeys = "id" | "name" | "email";

Next, we need to be able to exclude a specific string literal type from a union of string literal types. In the case of our User type, we want to exclude the type "email" from the union "id" | "name" | "email". We can use the Exclude<T, U> helper type to do that:

type UserKeysWithoutEmail = Exclude<UserKeys, "email">;

// This is equivalent to:
type UserKeysWithoutEmail = Exclude<
  "id" | "name" | "email",
  "email"
>;

// This is equivalent to:
type UserKeysWithoutEmail = "id" | "name";

The Exclude<T, U> type is defined in lib.es5.d.ts like this:

/**
 * Exclude from T those types that are assignable to U
 */
type Exclude<T, U> = T extends U ? never : T;

It's using a conditional type and the never type. Using the Exclude<T, U> helper type, we're removing those types in our union type "id" | "name" | "email" that are assignable to the "email" type. That is only true for the string literal type "email" itself, so we're left with the union type "id | "name".

Finally, we need to create an object type that contains a subset of the properties of our User type. Specifically, we want to create an object type that contains only those properties whose keys are found in the UserKeysWithoutEmail union type. We can use the Pick<T, K> helper type to pick those properties off of our User type:

type UserWithoutEmail = Pick<User, UserKeysWithoutEmail>;

// This is equivalent to:
type UserWithoutEmail = Pick<User, "id" | "name">;

// This is equivalent to:
type UserWithoutEmail = {
  id: string;
  name: string;
};

Here's how the Pick<T, K> helper type is defined within lib.es5.d.ts:

/**
 * From T, pick a set of properties whose keys are in the union K
 */
type Pick<T, K extends keyof T> = {
  [P in K]: T[P];
};

The Pick<T, K> type is a mapped type that's using the keyof operator and an indexed access type T[P] to retrieve the type of the property P in the object type T.

Now, let's summarize all the type operations we've performed using keyof, Exclude<T, U>, and Pick<T, K> in a single type:

type UserWithoutEmail = Pick<User, Exclude<keyof User, "email">>;

Notice that this type is specific to our User type. Let's make this a generic type so we can reuse it in other places:

type Omit<T, K> = Pick<T, Exclude<keyof T, K>>;

We can now use this type to compute our UserWithoutEmail type:

type UserWithoutEmail = Omit<User, "email">;

Since object keys can only be strings, numbers, or symbols, we can add a generic constraint to the type parameter K of our Omit<T, K> helper type to only allow types string, number, or symbol for keys:

type Omit<T, K extends string | number | symbol> =
  Pick<T, Exclude<keyof T, K>>;

The generic constraint extends string | number | symbol is a bit verbose. We can replace the string | number | symbol union type by the keyof any type since the two are equivalent:

type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>;

And there we go! We've arrived at the exact definition of the Omit<T, K> helper type as it is found within the lib.es5.d.ts type definition file:

/**
 * Construct a type with the properties of T except for those in type K.
 */
type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>;

Unrolling Omit<User, "email">

Here's a step-by-step evaluation of the Omit<User, "email"> type. Try to follow every step to understand how TypeScript is computing the final type:

type User = {
  id: string;
  name: string;
  email: string;
};

type UserWithoutEmail = Omit<User, "email">;

// This is equivalent to:
type UserWithoutEmail = Pick<
  User,
  Exclude<keyof User, "email">
>;

// This is equivalent to:
type UserWithoutEmail = Pick<
  User,
  Exclude<"id" | "name" | "email", "email">
>;

// This is equivalent to:
type UserWithoutEmail = Pick<
  User,
  | ("id" extends "email" ? never : "id")
  | ("name" extends "email" ? never : "name")
  | ("email" extends "email" ? never : "email")
>;

// This is equivalent to:
type UserWithoutEmail = Pick<User, "id" | "name" | never>;

// This is equivalent to:
type UserWithoutEmail = Pick<User, "id" | "name">;

// This is equivalent to:
type UserWithoutEmail = {
  [P in "id" | "name"]: User[P];
};

// This is equivalent to:
type UserWithoutEmail = {
  id: User["id"];
  name: User["name"];
};

// This is equivalent to:
type UserWithoutEmail = {
  id: string;
  name: string;
};

Et voilà, our final UserWithoutEmail type.

This post is part of the TypeScript Evolution series.