@nprindle/bob
v1.0.1
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Super typesafe, boilerplate-free builders for TypeScript
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bob
Super typesafe, boilerplate-free builders for TypeScript.
Motivation
Builders are a 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("[email protected]")
.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("[email protected]")
.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("[email protected]")
.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("[email protected]")
.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("[email protected]")
.id(999)
.age(20)
.build();
// Optional fields work as expected
const user2 = builder<User>()
.email("[email protected]")
.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("[email protected]")
.id(999)
.build();
To make a builder with default values, use builderDef
:
function userBuilder(): Builder<User, { id: number }> {
return builder({ id: 0 });
}
const user1 = userBuilder()
.email("[email protected]")
.build();
// Can override defaults if necessary
const user2 = userBuilder()
.email("[email protected]")
.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("[email protected]")
.id(999)
.build();