如何检查对象在JavaScript中是否具有特定属性? [英] How do I check if an object has a specific property in JavaScript?

问题描述

如何检查对象在JavaScript中是否具有特定属性?

How do I check if an object has a specific property in JavaScript?

考虑:

x = {'key': 1};
if ( x.hasOwnProperty('key') ) {
    //Do this
}

那是最好的方法吗?

推荐答案

我真的对给出的答案感到困惑-他们中的大多数完全是错误的.当然，您可以拥有具有未定义，空值或假值的对象属性.因此，简单地将属性检查减少为typeof this[property]或更糟糕的是，将使您完全误导结果.

I'm really confused by the answers that have been given - most of them are just outright incorrect. Of course you can have object properties that have undefined, null, or false values. So simply reducing the property check to typeof this[property] or, even worse, x.key will give you completely misleading results.

这取决于您要查找的内容.如果您想知道一个对象是否物理上包含一个属性(它不是来自原型链上的某个地方)，那么object.hasOwnProperty是可行的方法.所有现代的浏览器都支持它. (在Safari的较早版本(2.0.1和更早的版本中不存在)，但是那些浏览器版本已经很少使用了.)

It depends on what you're looking for. If you want to know if an object physically contains a property (and it is not coming from somewhere up on the prototype chain) then object.hasOwnProperty is the way to go. All modern browsers support it. (It was missing in older versions of Safari - 2.0.1 and older - but those versions of the browser are rarely used any more.)

如果您要查找的是对象上具有可迭代的属性(当您遍历该对象的属性时，它将出现)，则可以这样做:prop in object将为您提供所需的效果.

If what you're looking for is if an object has a property on it that is iterable (when you iterate over the properties of the object, it will appear) then doing: prop in object will give you your desired effect.

由于可能需要使用hasOwnProperty，并且考虑到可能需要后备方法，因此我向您提供以下解决方案:

Since using hasOwnProperty is probably what you want, and considering that you may want a fallback method, I present to you the following solution:

var obj = {
    a: undefined,
    b: null,
    c: false
};

// a, b, c all found
for ( var prop in obj ) {
    document.writeln( "Object1: " + prop );
}

function Class(){
    this.a = undefined;
    this.b = null;
    this.c = false;
}

Class.prototype = {
    a: undefined,
    b: true,
    c: true,
    d: true,
    e: true
};

var obj2 = new Class();

// a, b, c, d, e found
for ( var prop in obj2 ) {
    document.writeln( "Object2: " + prop );
}

function hasOwnProperty(obj, prop) {
    var proto = obj.__proto__ || obj.constructor.prototype;
    return (prop in obj) &&
        (!(prop in proto) || proto[prop] !== obj[prop]);
}

if ( Object.prototype.hasOwnProperty ) {
    var hasOwnProperty = function(obj, prop) {
        return obj.hasOwnProperty(prop);
    }
}

// a, b, c found in modern browsers
// b, c found in Safari 2.0.1 and older
for ( var prop in obj2 ) {
    if ( hasOwnProperty(obj2, prop) ) {
        document.writeln( "Object2 w/ hasOwn: " + prop );
    }
}

上面是hasOwnProperty的跨浏览器的可行解决方案，但有一个警告:无法区分原型和实例上具有相同属性的情况-只是假设它来自原型.您可以根据自己的情况将其更改为宽容或严格，但是至少这应该会有所帮助.

The above is a working, cross-browser, solution to hasOwnProperty, with one caveat: It is unable to distinguish between cases where an identical property is on the prototype and on the instance - it just assumes that it's coming from the prototype. You could shift it to be more lenient or strict, based upon your situation, but at the very least this should be more helpful.

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