json-stringify.html

<style>
  html textarea {
  min-height: 500px;
  width: 50%;
  margin: 0 10px;
  font-family: monospace;
}
</style>
<script>
    function convert (val) {
      document.querySelector('#b').value = JSON.stringify(val.trim())
    }
    function convert2 (val) {
      try {
        // negative lookahead to make sure not unescape string content
        const sanitized = val.replace(/(?<!\\)\\"/g,'"').replace(/(?<!\\)\\n/g, ' ')
        document.querySelector('#a').value = JSON.stringify(JSON.parse(sanitized), null, 2)

      } catch (e ) {
        console.log(e)
      }
    }
</script>
JSON stringify<br/>
<textarea name="" id="a" cols="30" rows="10" onchange="convert(this.value)"></textarea>
<br/>
JSON parse (and unescape)<br/>
<textarea name="" id="b" cols="30" rows="10" onchange="convert2(this.value)"></textarea>