toJson

impl.kt

fun <V> Map<String, V>.toJson() = convertObject(this)

private fun <V> convertObject(map: Map<String, V>) =
    "{ " + map.entries.joinToString(", ") { "\"${it.key}\": ${convertValue(it.value)}" } + " }"

private fun <V> convertValue(value: V): String = when (value) {
    is String -> "\"$value\""
    is List<*> -> "[ " + value.joinToString(", ") { convertValue(it) } + " ]"
    is Map<*, *> -> convertObject(value as Map<String, V>)
    else -> value.toString()
}

test.kt

import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.Test

object MapToJsonTest {

    @Test
    fun `test integer`() {
        assertEquals("""{ "a": 1 }""", mapOf("a" to 1).toJson())
    }

    @Test
    fun `test float`() {
        assertEquals("""{ "a": 1.1 }""", mapOf("a" to 1.1).toJson())
    }

    @Test
    fun `test null`() {
        assertEquals("""{ "a": null }""", mapOf("a" to null).toJson())
    }

    @Test
    fun `test bool`() {
        assertEquals("""{ "a": false }""", mapOf("a" to false).toJson())
    }

    @Test
    fun `test str`() {
        assertEquals("""{ "a": "str" }""", mapOf("a" to "str").toJson())
    }

    @Test
    fun `test array`() {
        assertEquals("""{ "a": [ 1, 2, 3 ] }""", mapOf("a" to listOf(1, 2, 3)).toJson())
    }

    @Test
    fun `test inner obj`() {
        val obj = mapOf(
            "a" to mapOf(
                "b" to 1
            )
        )
        assertEquals("""{ "a": { "b": 1 } }""", obj.toJson())
    }

    @Test
    fun `test array of objects`() {
        val obj = mapOf(
            "arr" to listOf(
                mapOf("x" to 1, "y" to 2),
                mapOf("z" to 3)
            )
        )
        assertEquals("""{ "arr": [ { "x": 1, "y": 2 }, { "z": 3 } ] }""", obj.toJson())
    }

    @Test
    fun `test complex obj`() {
        val obj = mapOf(
            "aString" to "str",
            "anObject" to mapOf(
                "bool1" to true,
                "theOther" to false
            ),
            "anArray" to listOf(1, 3),
            "imNull" to null
        )
        assertEquals(
            """{ "aString": "str", "anObject": { "bool1": true, "theOther": false }, "anArray": [ 1, 3 ], "imNull": null }""",
            obj.toJson()
        )
    }
}