blocks|key|2638905|text|第一个函数将告诉您是否定义了属性。它可以在没有定义的情况下存在。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2638906|第二个函数将告诉您对象是否直接具有该属性(但如果它通过原型继承该属性，则不会)。|2638907|第三个函数将告诉您该对象是否具有该属性。|2638908|第四个函数会告诉你是否有一个真值(+{+number_of_children:+0+}不是)。|offset|length|style|CODE|2638909|entityMap^0|0|0|0|I|P|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|P|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|Q|8|@$H|R|I|S|J|K]]|9|@]|A|$]]|$1|L|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|M|$]]

The first will tell you if a property is defined or not. It can exist without being defined.

The second will tell you if the object has the property directly (but not if it inherits it through the prototype).

The third will tell if you the object has the property at all.

The fourth will tell if you if the has a true value (which <code>{ number_of_children: 0 }</code> isn't).

blocks|key|4275003|text|这取决于情况，以及您希望您的测试有多严格。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4275004|4275005|不检查是否在特定对象上定义了somePropOrMethod属性；它只检查它是否在继承链中。通常，这并不重要；特别是在检查window.时，需要特别检查"somePropOrMethod“属性是否为而不是+undefined|ordered-list-item|offset|length|style|CODE|BOLD|4275006|4275007|函数Foo()+{}+Foo.prototype.bar+=+4；Foo.prototype.baz+=+undefined；var+x=+new+Foo()；typeof+x.bar​!==+"undefined"；//+true，但"x“实际上没有条形图属性；它在继承链类型x.baz​!==+"undefined"；//+false|4275008|检查特定对象是否持有该属性，并从搜索中排除继承链；但不检查somePropOrMethod是否未定义。|unordered-list-item|4275009|4275010|window.foo+=未定义；window.hasOwnProperty("foo")；//+true；但它未定义//+window.__proto__上的任何属性都将返回false|4275011|与第一个属性相同，除了它不检查undefined；它只检查对象有一些成员(可能是检查对象，以及它的继承链(即#1和#3区域)，但只检查真值；|4275012|4275013|window.foo+=+false；window.bar+=+0；window.baz+=+null；!!window.foo；//+false+!!window.bar；//+false+!!window.baz；//+false|4275014|4275015|当然，你应该做的是，如果你想要一个方法，你应该检查它是一个函数；|4275016|if(typeof+window.someMethod​+!==+"function"){+}+//+specifically+check+for+a+function|code-block|syntax|javascript|4275017|您应该对其他属性(string、number等，甚至使用instanceof关键字)执行相同的操作。|4275018|entityMap^0|0|0|E|G|1P|7|2V|9|2R|3|0|0|0|T|G|0|0|0|F|9|0|0|0|0|0|0|9|6|G|6|S|A|0^^$0|@$1|2|3|4|5|6|7|1A|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|1B|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|1C|8|@$F|1D|G|1E|H|I]|$F|1F|G|1G|H|I]|$F|1H|G|1I|H|I]|$F|1J|G|1K|H|J]]|9|@]|A|$]]|$1|K|3|-4|5|6|7|1L|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|1M|8|@]|9|@]|A|$]]|$1|N|3|O|5|P|7|1N|8|@$F|1O|G|1P|H|I]]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|1Q|8|@]|9|@]|A|$]]|$1|R|3|S|5|6|7|1R|8|@]|9|@]|A|$]]|$1|T|3|U|5|P|7|1S|8|@$F|1T|G|1U|H|I]]|9|@]|A|$]]|$1|V|3|-4|5|6|7|1V|8|@]|9|@]|A|$]]|$1|W|3|X|5|6|7|1W|8|@]|9|@]|A|$]]|$1|Y|3|-4|5|6|7|1X|8|@]|9|@]|A|$]]|$1|Z|3|10|5|6|7|1Y|8|@]|9|@]|A|$]]|$1|11|3|12|5|13|7|1Z|8|@]|9|@]|A|$14|15]]|$1|16|3|17|5|6|7|20|8|@$F|21|G|22|H|I]|$F|23|G|24|H|I]|$F|25|G|26|H|I]]|9|@]|A|$]]|$1|18|3|-4|5|6|7|27|8|@]|9|@]|A|$]]]|19|$]]

It depends on the situation, and how stringent you want your test to be.

<ol>
<li>Doesn't check that the <code>somePropOrMethod</code> attribute is defined on the specific object; it only checks it's in the inheritance chain. Normally this won't matter; especially when checking on <code>window.</code> It does however, specifically check the "somePropOrMethod" attribute is not <code>undefined</code>

<pre><code>function Foo() {

}
Foo.prototype.bar = 4;
Foo.prototype.baz = undefined;

var x = new Foo();

typeof x.bar​ !== "undefined"; // true, but "x" doesn't actually have a bar attribute; it's in the inheritance chain
typeof x.baz​ !== "undefined"; // false
</code></pre></li>
<li>Checks the whether the specific object holds the attribute and excludes the inheritance chain from it's search; but it doesn't check the <code>somePropOrMethod</code> isn't undefined.

<pre><code>window.foo = undefined;
window.hasOwnProperty("foo"); // true; but it's undefined

// any properties on window.__proto__ will return false
</code></pre></li>
<li>The same as the first, except it doesn't check against <code>undefined</code>; it just checks the object has some member (which could be undefined).</li>
<li>This checks the object, and it's inheritance chain (i.e. #1 and #3 territory), but is only checking for a truthy value;

<pre><code>window.foo = false;
window.bar = 0;
window.baz = null;

!!window.foo; // false
!!window.bar; // false
!!window.baz; // false
</code></pre></li>
</ol>

<hr>

Of course, what you should be doing is if you want a method, you should check it's a function;

<pre><code>if(typeof window.someMethod​ !== "function"){ } // specifically check for a function
</code></pre>

and you should do the same for other attributes (<code>string</code>, <code>number</code> etc, or even use the <code>instanceof</code> keyword).

blocks|key|4275208|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4275209|if(typeof+window.somePropOrMethod​+!==+"undefined"){+}|unordered-list-item|offset|length|style|CODE|4275210|4275211|检查somePropOrMethod是否是window的属性，并且它的值不是undefined。|4275212|直接在对象上以及在其原型链中检查|4275213|4275214|if(window.hasOwnProperty("somePropOrMethod")){+}+|4275215|4275216|检查somePropOrMethod是否显式存在于对象本身(window)上，而不管它的值如何(与#1不同)|4275217|仅在对象上直接检查|4275218|4275219|if("somePropOrMethod"+in+window){+}+|4275220|4275221|检查是否可以在对象本身或其原型链上找到somePropOrMethod，而不管它的值如何(与#1不同)|BOLD|4275222|4275223|4275224|if(!!window.somePropOrMethod)+{+}+|4275225|4275226|这里发生的事情是获取somePropOrMethod的值并将其转换为布尔值。如果somePropOrMethod为undefined或任何其他False值，则结果始终为false，否则为true|4275227|4275228|entityMap^0|0|0|1I|0|0|2|G|L|6|12|9|0|0|0|0|1C|0|0|2|G|U|6|0|0|0|0|Z|0|0|B|1|J|G|0|0|0|0|X|0|0|A|G|14|G|1L|9|2C|5|2L|4|0|0^^$0|@$1|2|3|-4|4|5|6|1B|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|1C|7|@$D|1D|E|1E|F|G]]|8|@]|9|$]]|$1|H|3|-4|4|5|6|1F|7|@]|8|@]|9|$]]|$1|I|3|J|4|5|6|1G|7|@$D|1H|E|1I|F|G]|$D|1J|E|1K|F|G]|$D|1L|E|1M|F|G]]|8|@]|9|$]]|$1|K|3|L|4|5|6|1N|7|@]|8|@]|9|$]]|$1|M|3|-4|4|5|6|1O|7|@]|8|@]|9|$]]|$1|N|3|O|4|C|6|1P|7|@$D|1Q|E|1R|F|G]]|8|@]|9|$]]|$1|P|3|-4|4|5|6|1S|7|@]|8|@]|9|$]]|$1|Q|3|R|4|5|6|1T|7|@$D|1U|E|1V|F|G]|$D|1W|E|1X|F|G]]|8|@]|9|$]]|$1|S|3|T|4|5|6|1Y|7|@]|8|@]|9|$]]|$1|U|3|-4|4|5|6|1Z|7|@]|8|@]|9|$]]|$1|V|3|W|4|C|6|20|7|@$D|21|E|22|F|G]]|8|@]|9|$]]|$1|X|3|-4|4|5|6|23|7|@]|8|@]|9|$]]|$1|Y|3|Z|4|5|6|24|7|@$D|25|E|26|F|10]|$D|27|E|28|F|G]]|8|@]|9|$]]|$1|11|3|L|4|5|6|29|7|@]|8|@]|9|$]]|$1|12|3|-4|4|5|6|2A|7|@]|8|@]|9|$]]|$1|13|3|14|4|C|6|2B|7|@$D|2C|E|2D|F|G]]|8|@]|9|$]]|$1|15|3|-4|4|5|6|2E|7|@]|8|@]|9|$]]|$1|16|3|17|4|5|6|2F|7|@$D|2G|E|2H|F|G]|$D|2I|E|2J|F|G]|$D|2K|E|2L|F|G]|$D|2M|E|2N|F|G]|$D|2O|E|2P|F|G]]|8|@]|9|$]]|$1|18|3|L|4|5|6|2Q|7|@]|8|@]|9|$]]|$1|19|3|-4|4|5|6|2R|7|@]|8|@]|9|$]]]|1A|$]]

<ul>
<li><code>if(typeof window.somePropOrMethod​ !== "undefined"){ }</code></li>
</ul>

checking if <code>somePropOrMethod</code> is a property of <code>window</code> and its value is anything but <code>undefined</code>. 
[ checking directly on the object aswell as in its prototype chain ]

<ul>
<li><code>if(window.hasOwnProperty("somePropOrMethod")){ }</code> </li>
</ul>

checking if <code>somePropOrMethod</code> exists explicitly on the object itself (<code>window</code>), regardless of its value (in difference to #1) 
[ checking only directly on the object]

<ul>
<li><code>if("somePropOrMethod" in window){ }</code> </li>
</ul>

checking if <code>somePropOrMethod</code> can be found on the object itself or its prototype chain, regardless of its value (in difference to #1) 
[ checking directly on the object aswell as in its prototype chain ]

<ul>
<li><code>if(!!window.somePropOrMethod) { }</code> </li>
</ul>

what happens here is to grab the value of <code>somePropOrMethod</code> and turn that into a boolean value. If <code>somePropOrMethod</code> is <code>undefined</code> or any other falsy value, the outcome will always be <code>false</code>, otherwise its <code>true</code> 
[ checking directly on the object aswell as in its prototype chain ]

I've seen many different ways of checking for available properties or methods in javascript.

<pre><code>if(typeof window.somePropOrMethod​ !== "undefined"){ }

if(window.hasOwnProperty("somePropOrMethod")){ }

if("somePropOrMethod" in window){ }

if(!!window.somePropOrMethod) { }
</code></pre>

Which one should I use, and why?
Is it all up to personal preference or are there subtle differences between them?

Best practice to check for available props or methods in javascript?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我见过许多不同的方法来检查javascript中可用的属性或方法。if(typeof window.somePropOrMethod​ !== "undefined"){ }if(window.hasOwnProperty("somePropOrMethod")){ }if("somePropOrMethod" in ...

问在javascript中检查可用的属性或方法的最佳实践？
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问在javascript中检查可用的属性或方法的最佳实践？EN