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isnative.js

// I recently saw http://davidwalsh.name/detect-native-function tweeted by
// @elijahmanor and was pretty jazzed about it. One of my favorite JS tricks is
// detecting native methods. Detecting native methods is handy because third
// party code can shim methods incorrectly as seen in past versions of
// Prototype.js, es5-shim, & modernizr, which can cause your code to behave in
// unexpected ways. This isn't a knock against those projects, shimming is really
// really hard to get right. Shimmed methods may also lack the performance
// benefits of their native counterparts. Lo-Dash, Dojo, Ember, & YUI,
// to name a few, detect native methods to avoid shims and rely on their own
// fallback paths, trusting their code over third-party.
//
// The technique, described in the blog post and used by most, is to use a regexp
// to detect `[native code]` from the function's string representation. This is
// a good start however it has some issues. This technique falls down when native
// method string representations don't contain `[native code]`. Granted nowadays
// most engines have standardized on `[native code]` but it is still non-spec'ed
// behavior. Another issue is that simply coercing a function to a string can be
// tricked by the function's `toString` method. So with those issues in mind lets
// examine how Lo-Dash detects native methods. The code below has been adapted
// from Lo-Dash's source.
;(function() {

  // Used to resolve the internal `[[Class]]` of values.
  var toString = Object.prototype.toString;
  
  // Used to resolve the decompiled source of functions.
  var fnToString = Function.prototype.toString;
  
  // Used to detect host constructors (Safari > 4; really typed array specific).
  var reHostCtor = /^\[object .+?Constructor\]$/;

  // Compile a regexp using a common native method as a template.
  // We chose `Object#toString` because there's a good chance it is not being mucked with.
  var reNative = RegExp('^' +
    // Coerce `Object#toString` to a string.
    String(toString)
    // Escape any special regexp characters.
    .replace(/[.*+?^${}()|[\]\/\\]/g, '\\$&')
    // Replace mentions of `toString` with `.*?` to keep the template generic.
    // Replace thing like `for ...` to support environments, like Rhino, which add extra
    // info such as method arity.
    .replace(/toString|(function).*?(?=\\\()| for .+?(?=\\\])/g, '$1.*?') + '$'
  );
  
  function isNative(value) {
    var type = typeof value;
    return type == 'function'
      // Use `Function#toString` to bypass the value's own `toString` method
      // and avoid being faked out.
      ? reNative.test(fnToString.call(value))
      // Fallback to a host object check because some environments will represent
      // things like typed arrays as DOM methods which may not conform to the
      // normal native pattern.
      : (value && type == 'object' && reHostCtor.test(toString.call(value))) || false;
  }
  
  // Export however you want.
  module.exports = isNative;
}());