Fluent Kotlin DSL for building JSON trees

JSObj.kt

@DslMarker
annotation class JSBuilderDsl

@DslMarker
annotation class JSSetterDsl

data class JSObj(val map: MutableMap<String, Any?> = mutableMapOf()) : MutableMap<String, Any?> by map {
    object Arr {
        @JSBuilderDsl
        operator fun get(vararg items: Any?) = items
    }

    @JSBuilderDsl
    val arr = Arr

    /**
     * set primitive
     */
    @JSSetterDsl
    infix fun String.by(value: Any?) {
        map[this] = value
    }

    /**
     * set object
     */
    @JSSetterDsl
    operator fun String.invoke(block: JSObj.() -> Unit) {
        map[this] = JSObj().apply(block)
    }

    /**
     * set array
     */
    @JSSetterDsl
    operator fun String.get(vararg items: Any?) {
        map[this] = items
    }
}

typealias JSArr<T> = Array<T>

/**
 * build array
 */
@JSBuilderDsl
fun <T> arr(vararg items: T): JSArr<out T> = items

/**
 * build object
 */
@JSBuilderDsl
fun obj(block: JSObj.() -> Unit): JSObj = JSObj().apply(block)

val myJsObject: JSObj = obj {
    "boolean" by true
    "number" by 1
    "string" by "str"
    "null" by null
    "array"[
        1,
        "str",
        null,
        obj {

        },
        arr[1, "2", null, 3]
    ]
    "object" {
        "boolean" by true
        "number" by 1
        "string" by "str"
        "null" by null
        "array"[1, "str", null]
    }
}