How to use recursion to turn object into string? EDIT
To me JSON.stringify is not the solution here, as it is much slower than recursion
    JSON.stringify(fakeData)
        .replace(/,/g, ';')
        .replace(/:(?={)/g, '')
        .replace(/&quot;/g, '')

//Result
Recursion with reduce x 816,321 ops/sec ±7.38% (80 runs sampled)
JSON.stringify x 578,221 ops/sec ±1.72% (92 runs sampled)
Fastest is Recursion with reduce

I am making a css-in-js library, and I need to use recursion to turn object into string like this:
Input:
const fakeData = {
  a: {
    b: 'c',
    d: 'e'
  }
}


Output:
a{b:c;d:e;}

My recursion function:
const buildKeyframe = (obj) =&gt; {
    return Object.entries(obj).reduce((acc, [prop, value]) =&gt; {
        if (typeof value === 'string') {
            return `${acc}${prop}:${value};`
        }

        return `${acc}${prop}:{${buildKeyframe(value)}}`
    }, '')
}

This function works, but I think there is room for improvement(e.g. use TCO, avoid using reduce)... How can I write a better function to recurse this data structure?
I agree that JSON.stringify isn't the most appropriate for this problem, not just because it's slower, but also because it's a bit harder to understand. And you're also right in assuming that reduce() isn't the best tool for the job. Using a .map() reads in a much more natural way.


const convert = obj =&gt; (
  Object.entries(obj)
    .map(([key, value]) =&gt; (
      typeof value === 'string'
        ? `${key}:${value};`
        : `${key}{${convert(value)}}`
    ))
    .join('')
)

console.log(convert({
  a: {
    b: 'c',
    d: 'e'
  },
}))



When optimizing this, think about the use case. While tail-call recursion may or may not be possible, it will make the code much more difficult to read, and it really won't help. The main advantage to tail-call recursion is to save memory on the call stack, and to let a recursive function call into itself an unlimited amount of times while still using just one call-frame worth of memory on the stack. There's no practical chunk of extremely nested CSS that can be written to make tail-call optimization worth it.
In your question, speed seems to be one of your primary concerns, but I'll play a bit of devil's advocate here and ask if it really should be for this use case. How much CSS-in-JS would you need to write before any of these micro-performance improvements even become noticeable? Tens of thousands of lines? Hundreds of thousands? I would suggest just building out the library first, run some benchmarks afterwards, and find out what parts of it actually run slow. Then only worry about fixing the slower areas.