Credit card detection

README.md

Returns the type of credit card for a given number (as string), recognizing Visa, MasterCard and American Express cards (data object can be easily extended with more card types!).

var whichCard = function(a,b,c){b={v:/^4\d{12}(?:\d{3})?$/,m:/^5[1-5]\d{14}$/,a:/^3[47]\d{13}$/};for(c in b)if(b[c].exec(a))return c};

whichCard('4111111111111111') // => 'v' (visa)

annotated.js

function(
  a,  // credit card number (string)
  b,  // placeholder
  c   // placeholder
) {
  b = {   // data object
    v: /^4\d{12}(?:\d{3})?$/,   // regexp for visa
    m: /^5[1-5]\d{14}$/,        // regexp for mastercard
    a: /^3[47]\d{13}$/          // regexp for american express
  };
  
  for (c in b)                  // for each card regexp
    if (b[c].exec(a))           // check if the number matches
      return c                  // if so, return the card type
}

index.js

function(a,b,c){b={v:/^4\d{12}(?:\d{3})?$/,m:/^5[1-5]\d{14}$/,a:/^3[47]\d{13}$/};for(c in b)if(b[c].exec(a))return c}

LICENSE.txt

Copyright (c) 2011 Thomas Fuchs, http://mir.aculo.us/
 
Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:
 
The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.
 
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

package.json

{
  "name": "creditcard_type",
  "description": "Returns the type of credit card for a given card number",
  "keywords": [
    "creditcard",
    "form"
  ]
}

Wouldn't adding a +"" inside the exec force the input to a string? still 120 bytes
function(a,b,c){b={v:/^4\d{12}(?:\d{3})?$/,m:/^5[1-5]\d{14}$/,a:/^3[47]\d{13}$/};for(c in b)if(b[c].exec(a+""))return c}