bob

Super typesafe, boilerplate-free builders for TypeScript.

• Motivation
• Usage

Motivation

Builders are very useful for declaratively constructing complex objects:

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

class UserBuilder {
    private obj: Partial<User> = {};

    setEmail(email: string): UserBuilder {
        this.obj.email = email;
        return this;
    }

    setId(id: number): UserBuilder {
        this.obj.id = id;
        return this;
    }

    build(): User {
        return this.obj as User;
    };
}

const user = new UserBuilder()
    .setEmail("foo@example.com")
    .setId(999)
    .build();

Unfortunately, there are a few problems with the above. Firstly, it's not very typesafe; if we don't provide all of the required fields, then we don't find out until runtime, due to the unsafe as User type assertion:

const user = new UserBuilder()
    .setEmail("foo@example.com")
    .build();
console.log(user.id); // undefined!

Fortunately, there are many ways that we can take advantage of TypeScript's structural type system to increase our type safety:

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

class UserBuilder implements Partial<User> {
    email?: string;
    id?: number;

    setEmail(email: string): this & Pick<User, "email"> {
        return Object.assign(this, { email });
    }

    setId(id: number): this & Pick<User, "id"> {
        return Object.assign(this, { id });
    }

    build(this: User): User {
        return {
            email: this.email,
            id: this.id
        };
    };
}

// Fails to compile, missing 'id'
const user = new UserBuilder()
    .setEmail("foo@example.com")
    .build();

Much better! However, this is a lot of boilerplate; it would be a pain to write a builder for all of our types. Libraries such as builder-pattern take advantage of Proxy to automatically create a builder for you, using the proxy's handler to set the necessary fields. This would let you write something like the following (using bob's syntax):

const user = builder<User>()
    .email("foo@example.com")
    .id(999)
    .build();

bob combines these two approaches, allowing you to make flexible and typesafe builders without all of the regular boilerplate.

Usage

To automatically make a builder for your type, use builder:

type User = {
    email: string;
    id: number;
    age?: number;
};

const user1 = builder<User>()
    .email("foo@example.com")
    .id(999)
    .age(20)
    .build();

// Optional fields work as expected
const user2 = builder<User>()
    .email("foo@example.com")
    .id(999)
    .build();

// Compile error! Builder missing required field 'email'
const user2 = builder<User>()
    .id(999)
    .build();

Of course, you can make a function to generate the builders of the desired type for you:

function userBuilder(): Builder<User> = {
    return builder<User>();
}

const user = userBuilder()
    .email("foo@example.com")
    .id(999)
    .build();

To make a builder with default values, use builderDef:

function userBuilder(): Builder<User, { id: number }> {
    return builderDef({ id: 0 });
}

const user1 = userBuilder()
    .email("foo@example.com")
    .build();

// Can override defaults if necessary
const user2 = userBuilder()
    .email("foo@example.com")
    .id(999)
    .build();

To make an instance out of the built fields, builder and builderDef can optionally accept a function as a second argument, which lets you specify how to construct an instance out of the fields you've built:

class User {
    constructor(
        public readonly email: string,
        public readonly id: number,
    ) {}

    // The first type parameter is the result of .build()
    // The second type parameter is the fields that the builder should have
    static builder(): Builder<User, { email: string; id: number; }> {
        return builder(({ email, id }) => new User(email, id));
    }
}

const user: User = User.builder()
    .email("foo@example.com")
    .id(999)
    .build();