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Matthias Hryniszak
Matthias Hryniszak

Posted on

Type-safe object builder in TS

Long story short - object builder pattern in TypeScript :)

type Builder<Source extends object, Target> = {
  [Prop in keyof Required<Source>]: (value: Source[Prop]) => Builder<Source, Target>
} & {
  build(): Target
}
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This type defines an object that will have methods of the same name as the Source object plus the build() method.

Here's an implementation thereof using Proxy:

function clone<S, T>(source: S): T {
  return JSON.parse(JSON.stringify(source))
}

function builder<
  Source extends object,
  Target
>(
  source: Source,
  convert: (source: Source) => Target = clone
): Builder<Source, Target> {

  const proxy = new Proxy<Builder<Source, Target>>(
    source as any, {
    get(target, prop: string, receiver) {
      if (prop === 'build') return build
      else return (value: any) => setter(
        prop as keyof Source, value
      )
    }
  });

  function build(): Target {
    return convert(source)
  }

  function setter(
    prop: keyof Source,
    value: any
  ): Builder<Source, Target> {
    source[prop] = value
    return proxy
  }

  return proxy
}
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And here's how you could use it:

const b = builder(
  {
    x: 1,
    y: 'Hello'
  },
  source => ({
    o1: { a: source.y, b: source.x },
    o2: { p1: source.x, p2: source.y },
  })
)

console.log(b.x(3).y('2').build())
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Each call to the builder methods is type-safe so it should be easy to work with it :)

Happy coding!

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