Javascript convert string to binary with a fixed length

Question

Is there an elegant way to convert some string to binary and get a result with a fixed length?
The built-in function is: parseInt(str, 10).toString(2), but it's cutting the length.

For example, if I want length = 8 bits, then myFunction("25") will return 00011001 instead of just 11001.

I know that I can append zeros to the beginning, but it doesn't seems like an elegant way to me.

Answer 1

Seems like the most elegant way to do that is to write (or get somewhere) missing abstractions and compose them to achieve desired result.

// lodash has this function
function padStart(string, length, char) {
  //  can be done via loop too:
  //    while (length-- > 0) {
  //      string = char + string;
  //    }
  //  return string;
  return length > 0 ?
    padStart(char + string, --length, char) :
    string;
}

function numToString(num, radix, length = num.length) {
  const numString = num.toString(radix);
  return numString.length === length ?
    numString :
    padStart(numString, length - numString.length, "0")
}

console.log(numToString(parseInt("25", 10), 2, 8));