在我的开发工作中经常会遇到需要判断一个函数是否是JavaScript原生函数的情况，有时候这是一个很必要的工作，你需要知道这个函数是浏览器自身提供的，还是由第三方封装、伪装成原生函数。当然，最好的方法是考察执行这个函数的toString方法的返回值。
The JavaScript
完成这个任务的方法非常简单：
更强悍的方法
Lodash的创始人John-David Dalton找到了一个更佳的方案:
// 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;
}());
