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When I want to convert a binary string to a base 10 decimal (Like this: parseInt('0001010', 2)), Javascript returns a decimal number, but a version in which the leading zeros mentioned in the example above have been disregarded. Is there any way to fix this?

jona
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  • ?? The result of that example should be the number 10. What did you expect instead? – Pointy Dec 27 '14 at 18:37
  • @Pointy what did he get, actually, the result of that number is ten –  Dec 27 '14 at 18:38
  • @NPE ha ha not enough/too much coffee :) :) – Pointy Dec 27 '14 at 18:39
  • @NPE ten. I've just run it on chrome console –  Dec 27 '14 at 18:39
  • See, if I try to parse `0001010011111101001010010`, it gives me 2751058, which, when reparsed through `str.toString(2)`, gives me `1010011111101001010010`. – jona Dec 27 '14 at 18:39
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    when going back to the original number it does not add the 0's at the left side. –  Dec 27 '14 at 18:40
  • huh. Then how exactly would I achieve that it does that? – jona Dec 27 '14 at 18:41
  • Why do you want to do so? If you need a fixed length of number, better count the number of digits and add the 0's on the left side instead. –  Dec 27 '14 at 18:42
  • @JuanRocamonde But what if I want to sort of shorten a long binary number, while being able to restore it to its original form later? – jona Dec 27 '14 at 18:43
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    Store the length in a variable and then, once restored, add the necessary number of 0's. –  Dec 27 '14 at 18:44
  • @kyr This is a problem of presentation. In base-10, we wouldn't write "four hundred" as 00000000400. Just pad with the number of zeroes you want in your UI / output. – Yuck Dec 27 '14 at 18:44
  • The problem I have is that I need to somehow fit everything in a URL query. – jona Dec 27 '14 at 18:45
  • @kyr check my answer. It might be helpful –  Dec 27 '14 at 18:49
  • Yeah, I figured so. Thanks, I'll accept that answer. – jona Dec 27 '14 at 18:50

2 Answers2

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The decimal representation has no way to keep track of leading zeros. If you wish to keep the leading zeros in the result, you need a fundamentally different approach (e.g. keeping the output as a string).

Alternatively, if you know the width of the result a priori, you could just pad it with leading zeros on output.

NPE
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So supposing you have the number '00000101101101':

var number = '00000101101101';
var length = number.length;
var decimal_number = parseInt(number, 2);

// going back

var new_number = decimal_number.toString(2);
var new_length = new_number.length;
var n_zeros = length - new_length;
var zeros = (n_zeros >= 2 ? Array(n_zeros+1).join("0") : "0");
new_number = zeros + new_number;
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    This is the right idea, but in JavaScript `0*n_zeros` doesn't do what the code expects it to do. [See this old question.](http://stackoverflow.com/questions/10073699/pad-a-number-with-leading-zeros-in-javascript) – Pointy Dec 27 '14 at 18:51
  • @Pointy so doesn't it put the string "0" `n` number of times? –  Dec 27 '14 at 18:52
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    No, it doesn't; `*` is always numeric multiplication. See the link I added to the first comment. – Pointy Dec 27 '14 at 18:52
  • @Juhana all right guys is it now working? I have just re-edited it –  Dec 27 '14 at 18:54