Add byteLength

README.md

@@ -21,8 +21,9 @@ With [npm](https://npmjs.org) do:
 
 `var base64 = require('base64-js')`
 
-`base64` has two exposed functions, `toByteArray` and `fromByteArray`, which both take a single argument.
+`base64` has three exposed functions, `byteLength`, `toByteArray` and `fromByteArray`, which both take a single argument.
 
+* `byteLength` - Takes a base64 string and returns length of byte array
 * `toByteArray` - Takes a base64 string and returns a byte array
 * `fromByteArray` - Takes a byte array and returns a base64 string
 

lib/b64.js

@@ -1,5 +1,6 @@
 'use strict'
 
+exports.byteLength = byteLength
 exports.toByteArray = toByteArray
 exports.fromByteArray = fromByteArray
 
@@ -20,10 +21,8 @@ function init () {
 
 init()
 
-function toByteArray (b64) {
-  var i, j, l, tmp, placeHolders, arr
+function placeHoldersCount (b64) {
   var len = b64.length
-
   if (len % 4 > 0) {
     throw new Error('Invalid string. Length must be a multiple of 4')
   }
@@ -33,9 +32,19 @@ function toByteArray (b64) {
   // represent one byte
   // if there is only one, then the three characters before it represent 2 bytes
   // this is just a cheap hack to not do indexOf twice
-  placeHolders = b64[len - 2] === '=' ? 2 : b64[len - 1] === '=' ? 1 : 0
+  return b64[len - 2] === '=' ? 2 : b64[len - 1] === '=' ? 1 : 0
+}
 
+function byteLength (b64) {
   // base64 is 4/3 + up to two characters of the original data
+  return b64.length * 3 / 4 - placeHoldersCount(b64)
+}
+
+function toByteArray (b64) {
+  var i, j, l, tmp, placeHolders, arr
+  var len = b64.length
+  placeHolders = placeHoldersCount(b64)
+
   arr = new Arr(len * 3 / 4 - placeHolders)
 
   // if there are placeholders, only get up to the last complete 4 chars

test/convert.js

@@ -13,7 +13,7 @@ var checks = [
 ]
 
 test('convert to base64 and back', function (t) {
-  t.plan(checks.length)
+  t.plan(checks.length * 2)
 
   for (var i = 0; i < checks.length; i++) {
     var check = checks[i]
@@ -25,6 +25,7 @@ test('convert to base64 and back', function (t) {
     str = map(arr, function (byte) { return String.fromCharCode(byte) }).join('')
 
     t.equal(check, str, 'Checked ' + check)
+    t.equal(b64.byteLength(b64Str), arr.length, 'Checked length for ' + check)
   }
 })