I found a JavaScript code in a gist whose purpose is to fix JSON strings. We can use it with the help of the V8 package to fix your JSON string.

Firstly, copy the JavaScript code below and save it under the name jsonNormalize.js:

// https://gist.github.com/dfkaye/dcb060a13470de8119833dc56933563f
// copyright David Kaye

/**
 * Helper functions and queries for normalize() method.
 */

const BLANK_LINES = /([n](s)*(?:n))/g;
const LINE_COMMENTS = /["']*(?:[sw]]*)(//[^n^"^}]*)/g;
// Attempt to handle /* */ and /** javadoc style * line * line2 comments */
const MULTILINE_COMMENTS = /["']*(?:s*)(/*+)([^*]*[*])([^/]*[/])/g;
const SINGLE_QUOTES = /([^\]['](?:[^']*)['](?:[^}^:^,]*))/g;
const UNQUOTED_WORDS = /(?:(,|:|{)s*)([w]+)/g;

function addQuotes(e) {

  // return booleans unchanged
  if (/false|true/.test(e)) {
    return e;
  }

  // return numbers unchanged
  if (/^d?.?d+$/.test(e)) {
    return e;
  }

  // replaces null, undefined, and NaN with empty string
  if (/null|undefined|NaN/.test(e)) {
    e = '';
  }

  return '"' + e + '"';
}

function replaceUnquoted(m) {
  return m.replace(/w+/g, addQuotes);
}

/**
 * @method normalize Attempts to fix a JSON string with bare keys (restore
 * missing quotes) and single-quoted keys and values, and remove line comments,
 * block comments, blank lines, etc.
 * @param {string} jsonText
 * @returns {string}
 */

function normalize(jsonText) {
  var fixed = jsonText

    // 18 Oct 2018 - remove comments and blank lines
    // 30 Oct 2018 - preserve quoted comments
    // remove line comments
    .replace(LINE_COMMENTS, e => {

      // Ignore if line comment is quoted.
      return /["']([sw]*[:])?/.test(e[0]) ? e : '';
    })

    // remove multi-line comments
    .replace(MULTILINE_COMMENTS, e => {

      // Ignore if comment is quoted.
      return /["']/.test(e[0]) ? e : '';
    })

    // remove blank lines
    .replace(BLANK_LINES, 'n')

    // 17,18 oct 2018
    // fix single quotes
    // 15 feb 2019
    // escaped apostrophes
    .replace(SINGLE_QUOTES, (m) => {

      /*
       * Replace leading and trailing single quotes with double quotes, trim
       * quoted spaces, ignore quoted apostrophes.
       */

      var t = m.trim();
      var p = m.split(t);
      var r = p[0] + t.replace(/^[']/, '"') + p[1];

      return r.replace(/(['])(?:[s]*)$/, (e) => {
        return '"' + (e.length > 1 ? e.substring(1) : '');
      });
    })

    // 17 october 2018
    // success
    // add missing quotes
    .replace(UNQUOTED_WORDS, replaceUnquoted)

    // 28 December 2019 - fix [01] and { 01: 01 }
    // e.g., replace 01 with "01"
    .replace(/(?=[^"^']+)[0]+([1-9)+(?=[^"^']+)/g, function (e) {
      return '"' + e + '"';
    })

    // trim it
    .trim();

  return fixed;
}

Now it is a child game to fix your JSON string with the V8 package:

x <- "[{area:30,ind:[5,5,4,5,4.14,3.75],isCluster:true},{area:31,ind:[],isCluster:false}]"

library(V8)
ctx <- v8()

ctx$source("jsonNormalize.js")
ctx$assign("x", x)
ctx$eval("var newx = normalize(x);")
ctx$get("newx")
# "[{"area":30,"ind":[5,5,4,5,4.14,3.75],"isCluster":true},{"area":31,"ind":[],"isCluster":false}]"

The double quotes are back!

Maybe this method deserves a small R package. It is useful. Thanks to the author of the gist, David Kaye.

Edit

I’ve just done the package

remotes::install_github("stla/jsonNormalize")
library(jsonNormalize)
# the keys of the following JSON string are not quoted
jstring <- "[{area:30,ind:[5,4.1,3.7],cluster:true},{ind:[],cluster:false}]"
jsonNormalize(jstring)
# [1] "[{"area":30,"ind":[5,4.1,3.7],"cluster":true},{"ind":[],"cluster":false}]"