Determine if string is in base64 using JavaScript

Question

I\'m using the window.atob(\'string\') function to decode a string from base64 to a string. Now I wonder, is there any way to check that \'string\' is actually

Answer 1

This should do the trick.

function isBase64(str) {
    if (str ==='' || str.trim() ===''){ return false; }
    try {
        return btoa(atob(str)) == str;
    } catch (err) {
        return false;
    }
}

Answer 2

As there are mostly two possibilities posted here (regex vs try catch) I did compare the performance of both: https://jsperf.com/base64-check/

Regex solution seems to be much faster and clear winner. Not sure if the regex catches all cases but for my tests it worked perfectly.

Thanks to @Philzen for the regex!

p.s.

In case someone is interested in finding the fastest way to safely decode a base64 string (that's how I came here): https://jsperf.com/base64-decoding-check

Answer 3

For me a string is likely an encoded base64 if:

1. it's length is divisible by 4
2. uses A-Z a-z 0-9 +/=
3. only uses = in the end (0-3 chars)

so the code would be

function isBase64(str)
{
    return str.length % 4 == 0 && /^[A-Za-z0-9+/]+[=]{0,3}$/.test(str);
}

Answer 4

I know its late, but I tried to make it simple here;

function isBase64(encodedString) {
    var regexBase64 = /^([0-9a-zA-Z+/]{4})*(([0-9a-zA-Z+/]{2}==)|([0-9a-zA-Z+/]{3}=))?$/;
    return regexBase64.test(encodedString);   // return TRUE if its base64 string.
}

Answer 5

Building on @atornblad's answer, using the regex to make a simple true/false test for base64 validity is as easy as follows:

var base64regex = /^([0-9a-zA-Z+/]{4})*(([0-9a-zA-Z+/]{2}==)|([0-9a-zA-Z+/]{3}=))?$/;

base64regex.test("SomeStringObviouslyNotBase64Encoded...");             // FALSE
base64regex.test("U29tZVN0cmluZ09idmlvdXNseU5vdEJhc2U2NEVuY29kZWQ=");   // TRUE

Answer 6

This is how it's done in one of my favorite validation libs:

const notBase64 = /[^A-Z0-9+\/=]/i;

export default function isBase64(str) {
  assertString(str); // remove this line and make sure you pass in a string
  const len = str.length;
  if (!len || len % 4 !== 0 || notBase64.test(str)) {
    return false;
  }
  const firstPaddingChar = str.indexOf('=');
  return firstPaddingChar === -1 ||
    firstPaddingChar === len - 1 ||
    (firstPaddingChar === len - 2 && str[len - 1] === '=');
}

https://github.com/chriso/validator.js/blob/master/src/lib/isBase64.js