Maxwell Kendall

TypeScript Tips and Tricks

Working with types in JavaScript is a lot easier when you have a few tips and tricks under your belt. This post goes over a few of them. The main theme is the introduction of basic classifications of types.

Prepended keywords

The Partial keyword allows you to specify that a given input or return is of a certain type, but may omit certain properties.

type User = {
    name: string,
    email: string
}

const myUser: Partial<User> = {
    name: 'Max'
};

This technique is preferable to defining another type w/ optional properties as the User type may change, in which case we would need to manage it as well as it's optional counterpart.

To acheive the opposite, by making optional types required, we can use the Required keyword:

type User = {
    name: string,
    email?: string
};

const myUser: Required<User> {
    name: 'max',
    email: 'test@test.com'
};

If we need to white list certain properties on a type we can use the Pick keyword:

type User = {
    name: string,
    email: string,
    gender: string,
    age: number
};

type UserPreview = ;

const myUser: Pick<User, "name" | "email"> {
    name: 'max',
    email: 'test@test.com'
};

If we need to black list certain properties on a type we can use the Omit keyword:

type User = {
    name: string,
    email: string,
    gender: string,
    age: number
};

const myUser: Omit<User, "gender", "age"> {
    name: 'max',
    email: 'test@test.com'
};

Immutable Variables and Types

Ready only variable:

const user = { 
    name: 'max'
} as const;

// this will error!
user.name = 'max kendall';
 

Type which enforces readonly variables after they are defined:

type User = {
    readonly name: string,
      // readonly favoriteBooks: Readonly<string[]>;
    readonly favoriteBooks: ReadonlyArray<string>;
    readonly education: {
        readonly degree: string
    }
}

const readOnlyUser: User = {
    name: 'Max',
    favoriteBooks: ['Moby Dick'],
    education: {
        degree: 'Bachelor of Arts'
    }
};

Types with Optional Properties

type User = {
    name: string,
    email?: string,
};

Since email is optional, we have to account for this when plucking the email property:

const user: User = {
    name: 'max',
    email: 'test@test.com'
}

// this errors
// const email: string = user.email;

// this works
const email: string = user.email!;

(this actually works w/o the ! in latest TypeScript)

Inferred Types

const user = {
  name: "John Doe",
  age: 27
};

type User = typeof user;

const user2: User = {
  age: 28,
  name: "Jane Doe"
};

Dynamically Typed Strings

    function handler(eventType: string) {
        console.log(`handling ${eventType}`);
    }

We want to enforce the prefix on as the input so only strings beginning with on will pass validation:

    
    function handler(eventType: `on${string}`) {
        console.log(`handling ${eventType}`);
    }

    handler('onClick');
    handler('test')

Line 6 fails validation, line 5 works.