Velocity Templating Language dealing with backslashes in input text

Question

Problem Statement

I'm in the process of writing an AWS API gateway and using Velocity Template Language (VTL) to transform my request into a structure that can be used by the backend. This has to be done on the API Gateway side since I have no control over the code that is calling the back, and the backend. The workflow is as follows:

-> legacy process sends request

-> API Gateway is invoked & transforms the request into a structure that is accepted by a lambda function

-> AWS lambda is invoked using transformed structure and returns a response

I have the template mostly figured out but there are a few niche cases surround special characters and I cannot find a solution that is universally able to resolve this.

Code

The transformation I have is as follows:

#set( $allParams = $input.params() )
#set( $InputBody = $util.escapeJavaScript($input.body) )
#set( $ParsedBody = $util.parseJson($input.body) )
#set( $text = $util.escapeJavaScript($ParsedBody.text) )
#set( $textString = $text.toString() )
#set( $text_language = $ParsedBody.language )
#set( $body = {
  "text": $textString,
  "categories": ["ALL"],
  "text_language": $text_language,
  "str_language_variants": ["en"]
})
#set($bodyString = $body.toString())
{
"body": "{\n  \"text\": \"$textString\",\n  \"text_language\": \"$text_language\",\n  \"str_language_variants\": [\"en\"], \n}",
}
}

Test Cases

1. The first test case has an inner \" which does not parse correctly

{
    "text": "besponsa sounds like \"famosa\" in spanish",
    "mode": "fb",
    "language": "en"
}

The error that I'm getting is:

Missing key arguments in the payloads ('text')

2. The second test case has a newline character in the text \n which with the above code is fine

{
    "text": "<i>Anxiety depression asthma Omicron</i> ad  \n asthma  anxiety, viagra, sildenafil, enzyme , mrtx1719",
    "mode": "all",
    "language": "en"
}

Attempted Solution(s) The only solution that has shown promise for the first case is if add a replace() method into the $textString to be as follows:

"body": "{\n  \"text\": \"$textString\"... -> "body": "{\n  \"text\": \"$textString.replace("\", "\\\"),\"

But this fails when I run test case 2.

Question

Is there anyway of having the VTL parse for both special character types while maintaining as much of the code above?