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Learn TypeScript generics by rebuilding existing utility types

Using TypeScript generic utilities to better understand constraints, conditional types and other generic type features

October 24, 2021 · 11 min read

TypeScript’s type system is very powerful because it allows us to express types in terms of other types. One way we do this is with generics, which are types that take parameters. A lot of times when we get started with generics, we actually use them directly within functions (see my previous post on Understanding TypeScript generics through lodash functions). But when we use them to create reusable types, we can create really powerful types much like the built-in utility types.

So I want to break down some existing TypeScript utilty types in order for us to better understand how TypeScript generics work. I personally learn best by seeing real-world examples. Somehow seeing something new in the context of something familiar helps me make the connection. I hoping it’ll do the same for you.

Without further ado, let’s dig in.


Basic generics

The Partial<Type> utility constructs a type with all of the properties of Type set to optional. This type is particularly useful when we have a function that will take a subset of data to update the full object.

For example:

interface Team {
  name: string
  city: string
  state: string
  since: Date
  value: number
  championships: number
}

// `fieldsToUpdate` can take any properties of a `Team` since
// all properties are optional. We don't have to pass in a
// full `Team` object
const updateTeam = (team: Team, fieldsToUpdate: Partial<Team>): Team => {
  return {
    ...team,
    ...fieldsToUpdate
  }
}

const before2021 = {
  name: 'Houston Rockets',
  city: 'Houston',
  state: 'Texas',
  since: new Date(1967, 1, 1),
  value: 2.5e9,
  championships: 2
}
const after2021 = updateTeam(before2021, {
  championships: 3,
  value: 3e9
})

In this example, we don’t want to have to pass an entirely new Team object to updateTeam in order to update just a few properties. All the properties of Team are required. So the fieldsToUpdate parameter uses Partial<Type> so that all the properties are now optional, allowing us to only pass in the championships and value fields.

As a result, the type definition of Partial<Type> is:

/**
 * Make all properties in `Type` optional
 */
type Partial<Type> = {
  [Key in keyof Type]?: Type[Key]
}

The <Type> indicates that the generic has a single parameter called Type. Using the keyof type operator, Partial<Type> first gets all of the properties of the generic Type. In the case of Partial<Team> that would be name, city, state, since, value, and championships. It then creates a new mapped type, mapping each property name to the original Type’s type for each property. The difference in this new mapped type is that the property values types also include undefined (via the optional property ?).

This could’ve also been written as:

/**
 * Make all properties in `Type` optional
 */
type Partial<Type> = {
  [Key in keyof Type]: Type[Key] | undefined
}

Generic types are kind of like type expressions or better yet implicit return functions. They can take 1 or more parameters (the generic types) and return a new type based on those parameters.

Required<Type> and Readonly<Type> work much in the same way. They toggle the required/readonly state on/off.

/**
 * Make all properties in `Type` required
 */
type Required<Type> = {
  [Key in keyof Type]-?: Type[Key]
}

/**
 * Make all properties in `Type` readonly
 */
type Readonly<Type> = {
  readonly [Key in keyof Type]: Type[Key]
}

Prefixing a modifier like ? or readonly with - means to remove the modifier. So -? removes the optional modifier, which removes undefined, making the field now required. With this information, we can make a fourth utility that doesn’t yet officially exist.

/**
 * Make all properties in `Type` *not* readonly (i.e. mutable)
 */
type Mutable<Type> = {
  -readonly [Key in keyof Type]: Type[Key]
}

We’re starting to build our generic type powers. 💥


Generic constraints

The Record<Keys, Type> utility constructs an object type whose propery keys are Keys and whose property values are Type. So this utility is used to create a map of the properties of a type to another type.

For example:

interface Player {
  firstName: string
  lastName: string
}
type Position = 'guard' | 'wing' | 'big'

const players: Record<Position, Player[]> = {
  guard: [
    { firstName: 'Kevin', lastName: 'Porter Jr.' },
    { firstName: 'Jalen', lastName: 'Green' },
  ],
  wing: [
    { firstname: 'Eric', lastName: 'Gordon' },
    { firstName: 'Danuel', lastName: 'House Jr.' },
    { firstName: "Jae'Sean", lastName: 'Tate' },
  ],
  big: [
    { firstName: 'Christian', lastName: 'Wood' },
    { firstName: 'Alperen', lastName: 'Sengun' },
    { firstName: 'Daniel', lastName: 'Theis' },
  ],
}

As a result, the type definition of Record<Keys, Type> is:

/**
 * Construct a type with a set of properties `Keys`
 * of type `Type`
 */
type Record<Keys extends string | number | symbol, Type> = {
  [Key in Keys]: Type
}

Record<Keys, Type> is mainly just used a shorthand for an object index signature. However, it adds a bit more power by using a mapped type instead (the Key in Keys part). The mapped type allows us to pass Position (a unioned literal type) for the object keys.

However, a JavaScript object can only support string, number or symbol keys, so Record<Keys, Type> uses a generic constraint (the extends string | number | symbol part) in order to restrict what types of keys are allowed. Trying to create a Record<boolean, Team> would fail because boolean doesn’t satisfy the constraint. But Position does satisfy the constraint because it is a subset of string.


Conditional generics

The NonNullable<Type> constructs a new type by excluding null and undefined from Type.

For example:

type NoUndef = NonNullable<Team | Player | undefined>
// NoUndef = Team | Player

type Cleansed = NonNullable<Player[] | undefined | null>
// Cleansed = Player[]

type Nothing = NonNullable<undefined | null>
// Nothing = never

As a result, the type definition of NonNullable<Type> is:

/**
 * Exclude `null` and `undefined` from `Type`
 */
type NonNullable<Type> = Type extends null | undefined ? never : Type

NonNullable<Type> uses a conditional type (the ternary operator) to do it’s magic. It goes through each sub-type in the union and sees if it extends null | undefined. If the sub-type is one of those 2, it replaces it with never. Otherwise, it returns the sub-type. The remaining sub-types are unioned together to construct the new type.

The never type is automatically omitted from any union type that has other sub-types (for instance Team | Player | never is just Team | Player). But if the type only has never (as in the case with Nothing in our example above), then the resultant type is also never.

The Exclude<Type, ExcludedUnion> type is actually a more generic version of NonNullable<Type>. It constructs a type by excluding from Type all union sub-types that are a part of ExcludedUnion.

For example:

type PlayersOnly = Exclude<Team | Player, Team>
// PlayersOnly = Player

type NoUndef = Exclude<Team | Player | undefined, null | undefined>
// NoUndef = Team | Player

type Cleansed = Exclude<Player[] | undefined | null, null | undefined>
// Cleansed = Player[]

type Nothing = Exclude<undefined | null, null | undefined>
// Nothing = never

Without even knowing how the type definition for Exclude is implemented, we should be able to rewrite NonNullable<Type> using Exclude<Type, ExcludedUnion>.

/**
 * Exclude `null` and `undefined` from `Type`
 */
type NonNullable<Type> = Exclude<Type, null | undefined>

That’s pretty cool if you ask me! It’s like generic composition.

The Extract<Type, Union> type is essentially the inverse of Exclude<Type, ExcludedUnion>. It constructs a new type by extracting from Type all the union sub-types that are a part of Union.

For example:

type SimpleOnly = Extract<Team | Player | boolean, number | boolean | string>
// SimpleOnly = boolean

As a result, the type definitions for Exclude<Type, ExcludedUnion> and Extract<Type, Union> are:

/**
 * Exclude from `Type` those types that are assignable to `ExcludedUnion`
 */
type Exclude<Type, ExcludedUnion> = Type extends ExcludedUnion ? never : Type

/**
 * Extract from `Type` those types that are assignable to `Union`
 */
type Extract<Type, Union> = Type extends Union ? Type : never

So Exclude<Type, ExcludedUnion> goes through each sub-type of Type and if the sub-type is a member of ExcludedUnion, never replaces that sub-type. All of the remaining sub-types are unioned together to return the new type.

And Extract<Type, Union> goes through each sub-type of its Type generic and if the sub-type is a member of Union, it’ll be included in the returned union.


Generic references

The Pick<Type, Keys> utility works exactly like the _.pick() lodash function except it returns a new type instead of a new object. It constructs a type by picking out the set of properties Keys from the specified Type. Keys can be a single string literal or a union of string literals.

For example:

interface Player {
  firstName: string
  lastName: string
}
type Position = 'guard' | 'wing' | 'big'
interface Team {
  name: string
  city: string
  state: string
  since: Date
  value: number
  championships: number
  players: Record<Position, Player[]>
}

type TeamMetadata = Pick<Team, 'name' | 'city' | 'state' | 'since'>
// TeamMetadata = {
//   name: string
//   city: string
//   state: string
//   since: Date
// }

As a result, the type definition for Pick<Type, Keys> is:

/**
 * From Type, pick a set of properties whose keys are in the union Keys
 */
type Pick<Type, Keys extends keyof Type> = {
  [Key in Keys]: Type[Key]
}

Things are starting to get interesting! Instead of the Keys generic being constrained on a known set of types (as was the case with Record<Keys, Type>), the Keys generic has a constraint based on the Type generic. The string literal or string literal union must extend from the keys of Type. Translation: Pick<Type, Keys> enforces that the strings we list are valid properties of the generic Type!

Pick<Type, Keys> then constructs a new subset object type with just the keys listed in Keys.

The Omit<Type, Keys> type constructs a new type by removing the specified Keys (either a string literal or string literal union) from Type.

For example:

interface Player {
  firstName: string
  lastName: string
}
type Position = 'guard' | 'wing' | 'big'
interface Team {
  name: string
  city: string
  state: string
  since: Date
  value: number
  championships: number
  players: Record<Position, Player[]>
}

type TeamMetadata = Omit<Team, 'value' | 'championships' | 'players'>
// TeamMetadata = {
//   name: string
//   city: string
//   state: string
//   since: Date
// }

The new TeamMetadata type is the same resultant type as with Pick<Type, Keys> but was constructed by omitting value, championships and players. So really an “omit” can be re-thought of as picking all of the properties of Type that are not the ones listed in Keys. Pick the ones excluded by Keys.

So it turns out we can actually build Omit<Type, Keys> from both Pick<Type, Keys> and Exclude<Type, ExcludedUnion>.

/**
 * Construct a type with the properties of `Type` except for those in type `Keys`.
 */
type Omit<Type, Keys extends keyof Type> = Pick<Type, Exclude<keyof Type, Keys>>

So first Omit<Type, Keys> uses Exclude<Type, ExcludedUnion> to create a union that has all the Keys removed from the properties of Type. So value, championships and players is removed from the properties of Team, leaving name, city, state, and since. The union of those remaining keys is then used to construct a new object with just those properties using Pick<Type, Keys>.

So again we were able to use generic composition to create more specific generics.


We’re really just scratching the surface with how TypeScript generics work by rebuilding the TypeScript’s utility types. Even I learned some new nuances about generics while writing this post!

And now that we better understand how generics work with constraints, conditions, and references, we’ll now be able to create our own generic types using the utilities as a base. We’ll deep dive into that in a future post.

But until then, did you find this helpful? Did it help demistify TypeScript generics a bit more? I hope so! Let me know by shooting me a tweet at @benmvp.

Keep learning my friends. 🤓

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Hi, I'm Ben Ilegbodu. 👋🏾

I'm a Christian, husband, and father of 3, with 15+ years of professional experience developing user interfaces for the Web. I'm a Principal Frontend Engineer at Stitch Fix, frontend development teacher, Google Developer Expert, and Microsoft MVP. I love helping developers level up their frontend skills.

Discuss on Twitter // Edit on GitHub