TypeSafeなbuilder

TypeSafeBuilder.ts

type DiffKey<T extends string, U extends string> = (
    & {[P in T]: P }
    & {[P in U]: never }
    & { [x: string]: never }
)[T];

type Omit<T, K extends keyof T> = Pick<T, DiffKey<keyof T, K>>

type Some = {
    x: string,
    y: string,
    z: string
}

type Builder<T, R = T> =
    & ({[K in keyof T]: (value: T[K]) => Builder<Omit<T, K>, R>})
    & ({ with: <K extends keyof T = keyof T>(value: T) => Builder<Omit<T, K>, R> })
    & ({ build: (this: Builder<{[x: string]: never}, R>) => R })



declare var builder: Builder<Some>

builder.x("zzz").y("yyy").z("dddd")

const some:Some = builder.x("vvv")
    .with({
        y: "ccc",
        z:"ddd"
    }).build()

const v = builder.x("x").y("y").z("zzz").build();