Boxed-Builder

A library to provide typesafe and reusuable utilities.

1. Builder pattern
2. Pipe pattern

Installation

Install with npm, yarn, or pnpm.

npm install boxed-builder
yarn install boxed-builder
pnpm install boxed-builder

Builder Pattern

A typesafe POJO-like builder, inspired by Vincent Pang's builder pattern.

Motivation

Often times, it's not possible (or ideal) to initialize all the required fields that describe an interface. For example,

1. Your application listens to incoming requests for data.
2. The code is "cleaner" if initialization is separated into multiple steps.

In TypeScript land, we usually hack around this problem using the keyword as or default values. However, both approaches are prone to costly runtime exceptions; i.e, the program inadvertently uses a field that has not been initialized.

Instead, you can utilize Builder to generate typesafe builder for your project.

import { Builder, Property } from "./boxed-builder";

class Shop {
  @Property
  name!: string;

  @Property
  open!: boolean;

  @Property
  stock!: number;

  @Property
  revenue!: number | undefined;
}

const shop = Builder(Shop)
  .Builder()
  .setName("Foo")
  .setOpen(true)
  .setStock(100)
  .setRevenue(0)
  .build();

console.log(shop); // { name: "Foo", open: true, stock: 100, revenue: 0 }

To avoid using code generation or ES6 Proxy, the builder is created by reading the properties from Shop. You will need to add @Property decorator because class properties do not exist until they are assigned to.

API

Out of the "box", Box(clazz) offers several strict builders. This means that an object can only be built if all the "required fields" are initalized. A field is non-required if its type can be undefined.

The builders differ in initialization pattern, but conform to the same API specification (described in the next section).

Builder	Description
Builder(clazz).Builder()	The most flexible builder type that has no constraints on how properties are set.
Builder(clazz).StagedBuilder<K extends Array<keyof clazz>>()	Requires properties to be set in the order specified by K.
Builder(clazz).ForwardBuilder()	Only allows forward initalization

Specification

Setter

Updating Value

For each property k in your POJO, the builder exposes a setter method set${k} to set the value for k. Note that the first character in k will be capitalized.

Builder(Shop).Builder().setLocation("Wonderland"); // Type error - Property 'setLocation' does not exist...
Builder(Shop).Builder().setOpen("true"); // Type error - "true" cannot be assigned to boolean
Builder(Shop).Builder().setOpen(false); // Returns a builder

Alternatively, you can supply a callback for the setter. The callback accepts an object with properties that have been initalized. This is useful if you want to set value conditionally.

Builder(Shop)
  .Builder()
  .setLocation(false)
  .setStock((shape) => {
    return shape.open ? 100 : 0;
  });

Validation

The setter also takes an optional callback to validate that the new value is sensible. The callback accepts an object with properties that have been initialized.

Builder(Shop)
  .Builder()
  .setStock(0)
  .setOpen(true, (shape) => {
    if (shape.open && shape.stock <= 0) {
      throw new Error("Can't open shop with no items");
    }
  });

Getter

For each property k that has been set, the builder exposes a getter method get${k}. Note that the first character in k will be capitalized.

Builder(Shop).Builder().getOpen(); // Type error - Property 'getOpen' does not exist...

Builder(Shop).Builder().setOpen(false).getOpen(); // false

From

You can partially initialize the values using from(other). Note that the builder only exposes from() on a fresh instance; i.e, you can't call from() on a builder instance that has fields set.

Builder(Shop)
  .Builder()
  .from({ open: true })
  .setStock(100, (shape) => {
    if (shape.open && shape.stock <= 0) {
      throw new Error("Can't open shop with no items");
    }
  });

Build

The builder exposes .build() method when all the required fields are set. A field is non-required if its type can be undefined.

Builder(shop).Builder().build(); // Type error - Property 'build' does not exist...

const shop = Builder(shop)
  .Builder()
  .setName("Foo")
  .setOpen(true)
  .setStock(100)
  .build(); // Ok since revenue has type number | undefined

console.log(shop); // { name: "Foo", open: true, stock: 100 }

Type Utility

You can convert a class to a type with Describe.

import { Describe } from "./boxed-builder";

type IShop = Describe<Shop>;

Staged Builder

You can enforce the initialization order using Builder(clazz).StagedBuilder<K extends Array<keyof clazz>>(). When all the properties in K have been set, a StagedBuilder converts to a regular builder.

Note that StagedBuilder only allows forward initalization on values in K. This means that if you call from() with properties in K, you can't re-initialize those values.

Builder(Shop).StagedBuilder<[]>(); // defaults to a regular builder

Builder(Shop).StagedBuilder<["name"]>().setLocation("MA"); // Type error - Property 'setLocation' does not exist...

Builder(Shop).StagedBuilder<["name"]>().setName("Foo"); // defaults to a regular builder after all stages are set

Forward Builder

You can enforce that properties are only initialized once using Builder(clazz).ForwardBuilder().

Builder(Shop).setLocation("Boston").setLocation("Boston"); // Type error - Property 'setLocation' does not exist...

Under The Hood

Builder uses conditional typing to enforce what methods are available to the client. However, it's possible to gain access to all the available API by downcasting and inspecting the builder object.

Pipe Pattern

A typesafe utility to compose functions, inspired by functional programming language.

Motivation

In-line function calls are very common.

f(g(h(x)));

For longer function names and multiple function calls, tracing parentheses can be difficult. Using Pipe API,

import { Pipe } from "./boxed-builder";

const pipe = Pipe(h).o(g).o(f);
pipe(x); // f(g(h(x)))

The code Pipe(h).o(g).o(f) roughly translates to

(x) => {
  let x1 = h(x);
  let x2 = g(x1);
  let x3 = f(x2);
  return x3;
};

Specification

A Pipe is an immutable list of unary functions. When a Pipe is called with a parameter, the functions are executed in the order they are added.

Creating Pipe

To create a Pipe

const isPositivePipe = Pipe((x: number) => x > 0);

To add more functions to the pipe, use the .o(unary) method

const isNotPositivePipe = isPositivePipe.o((x) => !x);

Calling .o() method returns a new Pipe instance.

To execute the pipe, call it as you would with a function. The output type is inferred by typechecking the list of unary functions in order.

isNotPositivePipe(-10); // true

Type Checking

The first function added to the Pipe can take any input. Any subsequent function takes the type returned by the previous function.

Pipe((x: number) => x * 10).o((y /* inferred as number */) => 10 + y)(1); // (1 * 10) + 10

Pipe((x: number) => x > 0).o((y: number) => -1 * y); // Type error, number is not assignable to boolean

Pipe((x: number) => x > 0)(true); // Type error

Asynchronous Function

If a function returns a promise, the Pipe waits for the results before calling the next function. The output of the pipe will be a promise.

const x = await Pipe(async (x: number) => x + 1)(10); // 1

Terminating

It may be useful to terminate the chain early; for example, when an error occurred. The unary functions can optionally take in a terminate function, which has type (output?: any) => never.

const pipe = Pipe((student: { school: string | undefined }, terminate) => {
  if (student.school == undefined) {
    return terminate("STOP");
  }
  // student.school is inferred to be string
  return student.school === "Tufts";
});

pipe({ school: undefined }); // STOP
pipe({ school: "Tufts" }); // true

The parameter to terminate becomes the output of the pipe. This can be useful for error recovery.