Reversed Binary Numbers in JavaScript

Question

I'm trying it out just to see if I could solve it. It would be great if I could receive tips on how to improve it.

//input
var decimal = prompt("Please enter a decimal number");
if (decimal >= 1 && decimal < 1000000000) {
    //convert to binary
    var binConvert = parseInt(decimal, 10).toString(2);

    //reverse binary number
    var makeString = binConvert.toString();
    var srj = makeString.split("").reverse().join("");
    var makeNumber = Number(srj);

    //convert back to decimal
    alert(parseInt(makeNumber, 2));

} else {
    alert("Input number should be: 1 <= N < 1 000 000 000. Please try again.");
}

jsFiddle

Answer 1

prompt("Please enter a decimal number");

is a little confusing to me. I thought you wanted a number with a few decimal digits on it.

It might make more sense to say "Please enter a base 10 number"

---

You are a little incosistent in how you turn your string numbers into number numbers.

In the beginning, you use parseInt:

var binConvert = parseInt(decimal, 10).toString(2);

In the end, you use parseInt:

alert(parseInt(makeNumber, 2));

But in the middle, you use Number:

var makeNumber = Number(srj);

It is better to be consistent and stick to one function (in my opinion, you should use parseInt).

---

If you take the above advice in using parseInt, you can then get rid of the last parseInt call like so:

var makeNumber = parseInt(srj, 2); // I moved the parseInt call up here

//convert back to decimal
alert(makeNumber);

---

This might just be me, but your comments are a little confusing.

Here, you wrote:

//convert back to decimal
alert(parseInt(makeNumber, 2));

Aren't you technically converting to binary? (The radix parameter of parseInt is 2).

Answer 2

SirPython already pointed out what I would have, so this is a wholly different approach rather than a review.

Instead of doing number-to-string-to-number conversions, we can employ some bitwise voodoo:

var input = prompt("Please enter a positive decimal number"),
    decimal = parseInt(input, 10);

if(decimal > 0 && decimal < 1000000000) {
  
  var reversed = decimal & 1; // set the least-significant bit (LSB)
  
  // loop through remaining bits
  while(decimal > 1) {
    reversed <<= 1; // shift a zero bit onto "the end" of reversed
    decimal  >>= 1; // shift the LSB off of decimal
    reversed += decimal & 1; // add decimal's new LSB
  }
  
  alert(reversed);

} else {
  alert("Input number should be: 0 < N < 1 000 000 000.");
}

Basically, each bit we shift off of decimal gets shifted onto reversed in, well, in reverse.

By the way: It can only handle input up to 2,147,483,647 (i.e. signed 32-bit integer max). Bitwise operations in JavaScript start misbehaving after that. I've just kept your current limit, though.