JSON to CSS in JavaScript
The main problem I see is that it's not obvious what the output is because there's too much auxiliary code that effectively obfuscates the logic.


Offload some of the code into a function.
Use template strings.


If the data amount isn't in multi-megabyte range I would write a single string to improve readability:

const dataJson = JSON.parse(data);
const breakpointMap = dataJson['breakpoints-map'];
delete dataJson['breakpoints-map'];

const entryToCss = ([mapKey, mapValues], breakpoint) =&gt; {
  const property = functions.getProperty(mapKey);
  return Object.entries(mapValues)
    .map(([k, v]) =&gt; `.${breakpoint}${k}{ ${property}: ${v}}\n`)
    .join('');
};

cssWriter.write(
  Object.entries(breakpointMap).map(([bpKey, bpVal]) =&gt; {
    const breakpoint = functions.isDefault(bpKey) ? `${bpKey}\\:` : '';
    return `${
      breakpoint ? `@media ${bpVal} {\n` : ''
    }${
      Object.entries(dataJson)
        .map(entry =&gt; entryToCss(entry, breakpoint))
        .join('')
    }${
      breakpoint ? '}\n' : ''
    }`;
  }).join('')
);