jQuery's isFunction and InternetExplorer

问题:

When adapting jQuery's isFunction method

isFunction: function( obj ) {
  return toString.call(obj) === "[object Function]";
}

InternetExlorer 8 returns the following error: "Object doesn't support this property or method". A good article digging into this function won't fix this behaviour. To check wether obj is defined, I changed the function in reference to the MSDN article:

isFunction: function( obj ) {
  return obj && toString.call(obj) === "[object Function]";
}

Any other ideas for a solution?

回答1:

You need to call the toString method directly from the Object.prototype object:

function isFunction (obj) {
    return Object.prototype.toString.call(obj) === "[object Function]";
}

alert(isFunction({})); // false
alert(isFunction(function{})); // true

jQuery has a local variable named toString that refers to the method on Object.prototype.

Calling just toString.call(obj); without declaring a toString identifier on scope, works on Firefox, Chrome, etc, just because the Global object inherits from Object.prototype, but this is not guaranteed by the spec.

Object.prototype.isPrototypeOf(window); // false on IE

The article you link to talks about a change that was introduced in the ECMAScript 5th Edition specification to the call and apply methods.

Those methods allow you to invoke a function passing the first argument as the this value of the invoked function.

On ECMAScript 3, if this argument was undefined or null, the this value of the invoked function will refer to the global object, for example:

function test () {  return this; }
test.call(null) === window; // true

But that changed in ES5, the value should now be passed without modification and that caused the Object.prototype.toString to throw an exception, because an object argument was expected.

The specification of that method changed, now if the this value refers to undefined or null the string "[object Undefined]" or "[object Null]" will be returned, fixing the problem (thing that I don't think is really good, since both results results feel just wrong, undefined and null are not objects, they are primitives... that made me remember typeof null == 'object'... Moreover I think it messes up with the concept of the [[Class]] internal property, anyway...)

Now you might wonder why jQuery uses this method to check for a function object, instead of using the typeof operator?

Implementation bugs, for example in Chrome/Safari/WebKit, the typeof for RegExp objects returns "function", because RegExp objects where made callable, e.g.:

typeof /foo/; // "function" in Chrome/Safari/WebKit

The typeof operator returns "function" if the object implements the [[Call]] internal property, which made objects to be callable.

This was originally introduced by the Mozilla implementations, invoking a RegExp object is equivalent to call the exec method.

回答2:

What purpose is the isFunction function trying to serve? Is it intended to deal with host objects (i.e. DOM nodes, window etc. in browsers), or regular expressions (which as CMS points out in the comments are callable and return "function" with typeof in some browsers)? If not, all you need is

typeof obj == "function"

If it is supposed to deal with host methods, then it's flawed because there's no obligation that the browser must return "[object Function]" for host methods. In IE there are many callable host properties that won't return "[object Function]" or will throw an error. For example:

alert(Object.prototype.toString.call(document.createElement));

... gives "[object Object]" in IE.

回答3:

I just ran this in IE 8's console:

>>$.isFunction(undefined);
false

Works fine.

The blog post you reference concerns IE 9.