<div data-a="true" data-b="false" data-d="5" data-e="null"></div>

<input type="text" value="true">

<script>

    var rbrace = /(?:\{[\s\S]*\}|\[[\s\S]*\])$/

    function parseData(data) {

        //console.log(data)

        try {

            if (typeof data === "object")

                return data

            data = data === "true" ? true :

                    data === "false" ? false :

                    data === "null" ? null :

                    +data + "" === data ? +data :

                    rbrace.test(data) ? JSON.parse(data) : data

        } catch (e) {

        }

        return data

    }

    var dataset = document.querySelector('div').dataset

    //console.log(parseData(dataset.a)) // "true" true

    //console.log(parseData(dataset.b)) // "false" false

    //console.log(parseData(dataset.c)) // undefined undefined

    //console.log(dataset.e, parseData(dataset.e)) // "null" null

    //console.log(parseData(dataset.d)) // "5" 5

</script>