jquery的isPlainObject检查是不是对象例子

对象一词经常在日常生活中使用，在js中对象{}也是很常用，于是得有个“检查是不是对象”的方法吧，然后jquery提供isPlainObject方法。。。

//jquery1.11.2
isPlainObject: function( obj ) {
    var key;

    // Must be an Object.
    // Because of IE, we also have to check the presence of the constructor property.
    // Make sure that DOM nodes and window objects don't pass through, as well
    if ( !obj || jQuery.type(obj) !== "object" || obj.nodeType || jQuery.isWindow( obj ) ) {
        return false;
    }

    try {
        // Not own constructor property must be Object
        if ( obj.constructor &&
            !hasOwn.call(obj, "constructor") &&
            !hasOwn.call(obj.constructor.prototype, "isPrototypeOf") ) {
            return false;
        }
    } catch ( e ) {
        // IE8,9 Will throw exceptions on certain host objects #9897
        return false;
    }

    // Support: IE<9
    // Handle iteration over inherited properties before own properties.
    if ( support.ownLast ) {
        for ( key in obj ) {
            return hasOwn.call( obj, key );
        }
    }

    // Own properties are enumerated firstly, so to speed up,
    // if last one is own, then all properties are own.
    for ( key in obj ) {}

    return key === undefined || hasOwn.call( obj, key );
},

测试用例是：

asyncTest("isPlainObject", function() {
    expect(15);

    var pass, iframe, doc,
        fn = function() {};

    // The use case that we want to match
    ok( jQuery.isPlainObject({}), "{}" );

    // Not objects shouldn't be matched
    ok( !jQuery.isPlainObject(""), "string" );
    ok( !jQuery.isPlainObject(0) && !jQuery.isPlainObject(1), "number" );
    ok( !jQuery.isPlainObject(true) && !jQuery.isPlainObject(false), "boolean" );
    ok( !jQuery.isPlainObject(null), "null" );
    ok( !jQuery.isPlainObject(undefined), "undefined" );

    // Arrays shouldn't be matched
    ok( !jQuery.isPlainObject([]), "array" );

    // Instantiated objects shouldn't be matched
    ok( !jQuery.isPlainObject(new Date()), "new Date" );

    // Functions shouldn't be matched
    ok( !jQuery.isPlainObject(fn), "fn" );

    // Again, instantiated objects shouldn't be matched
    ok( !jQuery.isPlainObject(new fn()), "new fn (no methods)" );

    // Makes the function a little more realistic
    // (and harder to detect, incidentally)
    fn.prototype["someMethod"] = function(){};

    // Again, instantiated objects shouldn't be matched
    ok( !jQuery.isPlainObject(new fn()), "new fn" );

    // DOM Element
    ok( !jQuery.isPlainObject( document.createElement("div") ), "DOM Element" );

    // Window
    ok( !jQuery.isPlainObject( window ), "window" );

    pass = false;
    try {
        jQuery.isPlainObject( window.location );
        pass = true;
    } catch ( e ) {}
    ok( pass, "Does not throw exceptions on host objects" );

    // Objects from other windows should be matched
    Globals.register("iframeDone");
    window.iframeDone = function( otherObject, detail ) {
        window.iframeDone = undefined;
        iframe.parentNode.removeChild( iframe );
        ok( jQuery.isPlainObject(new otherObject()), "new otherObject" + ( detail ? " - " + detail : "" ) );
        start();
    };

    try {
        iframe = jQuery("#qunit-fixture")[0].appendChild( document.createElement("iframe") );
        doc = iframe.contentDocument || iframe.contentWindow.document;
        doc.open();
        doc.write("");
        doc.close();
    } catch(e) {
        window.iframeDone( Object, "iframes not supported" );
    }
});

以上判断的有dom,window,{},function,null,undefined,new
Object,Number,String,Boolean,Array,Date等，但那么问题来了：

var obj = {};//这是一个对象吧？
console.log($.isPlainObject(obj));//true
var obj2 = {"xuexb": 123};//这也是一个对象吧？
console.log($.isPlainObject(obj2));//true
var obj3 = {"nodeType": "啦啦啦"};//这也是一个对象吧？
console.log($.isPlainObject(obj3));//false

怎么回事呢？ 其实原因就在if ( !obj || jQuery.type(obj) !== "object" || obj.nodeType || jQuery.isWindow( obj ) ) {这一判断，jquery认为只要对象的key里有nodeType就是dom，那么俺的对象就不能叫这个了？当然jquery这么做肯定有她的原因，只是俺不知道罢了