Detect if a Function is Native Code with JavaScript

Every once a while I'll test is a given function is native code -- it's an important part of feature testing whether a function was provided by the browser or via a third party shim which acts like the native feature.  The best way to detect this, of course, is evaluating the toString return value of the function.

The JavaScript

The code to accomplish this task is fairly basic:

function isNative(fn) {
	return (/\{\s*\[native code\]\s*\}/).test('' + fn);
}

Converting to the string representation of the function and performing a regex match on the string is how it's done.  There isn't a better way of confirming a function is native code!

Update!

Lodash creator John-David Dalton has provided a better solution:

;(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;
}());

So there you have it -- a better solution for detecting if a method is native. Of course you shouldn't use this as a form of security -- it's only to hint toward native support!