blocks|key|3527766|text|在JavaScript中，如果您忽略了提供一个参数，它将缺省为undefined。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3527767|您可以在自己的浏览器控制台中或使用JSFiddle轻松地尝试它。|3527768|正如您所说，您可以检查参数是否存在，这样就可以编写一个可以使用参数或不使用参数的函数。然而，JavaScript+Garden+(一个很棒的资源)建议在大多数其他情况下远离typeof，因为它的输出几乎是无用的(请查看results+of+typeof的表格)。|3527769|entityMap|0|LINK|mutability|MUTABLE|url|http://jsfiddle.net/|1|http://bonsaiden.github.com/JavaScript-Garden/|2|http://bonsaiden.github.com/JavaScript-Garden/#types.typeof^0|V|9|0|H|8|0|0|2E|6|1A|H|1|31|H|2|0^^$0|@$1|2|3|4|5|6|7|V|8|@$9|W|A|X|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Y|8|@]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|H|3|I|5|6|7|12|8|@$9|13|A|14|B|C]]|D|@$9|15|A|16|1|17]|$9|18|A|19|1|1A]]|E|$]]|$1|J|3|-4|5|6|7|1B|8|@]|D|@]|E|$]]]|K|$L|$5|M|N|O|E|$P|Q]]|R|$5|M|N|O|E|$P|S]]|T|$5|M|N|O|E|$P|U]]]]

In JavaScript, if you neglect to give a parameter it will default to <code>undefined</code>.

You could try it out for yourself easily enough, either in your browser console or using <a href="http://jsfiddle.net/" rel="noreferrer">JSFiddle</a>.

You can check for the existance of the parameter, as you say, and that way write a function that can use a parameter or not. However, <a href="http://bonsaiden.github.com/JavaScript-Garden/" rel="noreferrer">JavaScript Garden</a> (a great resource) recommends staying away from <code>typeof</code> in most other cases, as its output is just about useless (check out the table of <a href="http://bonsaiden.github.com/JavaScript-Garden/#types.typeof" rel="noreferrer">results of typeof</a>).

blocks|key|3044450|text|这是一种非常常见的模式。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3044451|您可以使用以下命令测试它|3044452|function+parameterTest(bool)+{
++if+(bool+!==+undefined)+{|code-block|syntax|javascript|3044453|然后，您可以使用这些形式之一调用您的函数：|3044454|+parameterTest();
+parameterTest(someValue);|3044455|注意不要造成频繁的测试错误|3044456|if+(!bool)+{|3044457|因为您无法将未提供的值与false、0或""区分开来。|offset|length|style|CODE|3044458|entityMap^0|0|0|0|0|0|0|0|C|5|I|1|K|2|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|10|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|11|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|12|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|13|8|@]|9|@]|A|$]]|$1|O|3|P|5|F|7|14|8|@]|9|@]|A|$G|H]]|$1|Q|3|R|5|6|7|15|8|@$S|16|T|17|U|V]|$S|18|T|19|U|V]|$S|1A|T|1B|U|V]]|9|@]|A|$]]|$1|W|3|-4|5|6|7|1C|8|@]|9|@]|A|$]]]|X|$]]

This is a very frequent pattern.

You can test it using

<pre><code>function parameterTest(bool) {
 if (bool !== undefined) {
</code></pre>

You can then call your function with one of those forms :

<pre><code> parameterTest();
 parameterTest(someValue);
</code></pre>

Be careful not to make the frequent error of testing

<pre><code>if (!bool) {
</code></pre>

Because you wouldn't be able to differentiate an unprovided value from <code>false</code>, <code>0</code> or <code>""</code>.

blocks|key|5166211|text|function+parameterTest(bool)
{
+++if(typeof+bool+!==+'undefined')
+++{
+++++alert('the+parameter+exists...');
+++}
+++else
+++{
+++++alert('The+parameter+doesn\'t+exist...');
+++}
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|5166212|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>function parameterTest(bool)
{
 if(typeof bool !== 'undefined')
 {
 alert('the parameter exists...');
 }
 else
 {
 alert('The parameter doesn\'t exist...');
 }
}
</code></pre>

blocks|key|3044477|text|function+parameterTest(p)+{
++++if+(+p+===+undefined)
++++++++alert('The+parameter+doesn\'t+exist...');
++++else
++++++++alert('the+parameter+exists...');
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|3044478|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre class="lang-js prettyprint-override"><code>function parameterTest(p) {
 if ( p === undefined)
 alert('The parameter doesn\'t exist...');
 else
 alert('the parameter exists...');
}
</code></pre>

blocks|key|3044558|text|检查的最佳方法:如果参数不是未定义的|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|3044559|function+parameterTest(param)+{
++++if+(param+!==+undefined)
++++...|code-block|syntax|javascript|3044560|参数也可以是变量或函数名|3044561|entityMap^0|A|2|E|4|0|0|0|2|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|N|$]]

best way to check: if the param is not undefined

<pre><code>function parameterTest(param) {
 if (param !== undefined)
 ...
</code></pre>

the param could be also a variable or a function name

blocks|key|3527895|text|我知道这很旧，但这是我首选的检查方法，并将默认值分配给函数：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3527896|function+testParamFunction(param1,+param2)+{
++++param1+=+typeof+param1+===+'undefined'+?+null+:+param1;
++++param2+=+typeof+param2+===+'undefined'+?+'default'+:+param2;

++++//+exit+if+the+required+parameter+is+not+passed
++++if+(param1+===+null)+{
++++++++console.error('Required+parameter+was+not+passed');
++++++++return;
++++}

++++//+param2+is+not+mandatory+and+is+assigned+a+default+value+so+
++++//+things+continue+as+long+as+param1+has+a+value
}|code-block|syntax|javascript|3527897|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I know this is old, but this is my preferred way to check, and assign default values to functions:

<pre><code>function testParamFunction(param1, param2) {
 param1 = typeof param1 === 'undefined' ? null : param1;
 param2 = typeof param2 === 'undefined' ? 'default' : param2;

 // exit if the required parameter is not passed
 if (param1 === null) {
 console.error('Required parameter was not passed');
 return;
 }

 // param2 is not mandatory and is assigned a default value so 
 // things continue as long as param1 has a value
}
</code></pre>

blocks|key|3044605|text|null+==+undefined为真|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3044606|if+(arg+==+null){
++++//+arg+was+not+passed.
}|code-block|syntax|javascript|3044607|代码示例：|3044608|​|3044609|var+button+=+document.querySelector("button");

function+myFunction(arg){
++if(arg+==+null){
++++alert("argument+was+not+passed.");
++}+else+{
++++alert("argument+"+%2B+arg+%2B+"+was+passed.");
++}
}|3044610|<button+onclick="myFunction('foo');">click+to+fire+function+w+arg</button>
 
<button+onclick="myFunction();">click+to+fire+function+w/o+arg</button>|3044611|3044612|entityMap^0|0|H|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|V|8|@$9|W|A|X|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|Y|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Z|8|@]|D|@]|E|$]]|$1|M|3|N|5|6|7|10|8|@]|D|@]|E|$]]|$1|O|3|P|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|Q|3|R|5|H|7|12|8|@]|D|@]|E|$I|J]]|$1|S|3|N|5|6|7|13|8|@]|D|@]|E|$]]|$1|T|3|-4|5|6|7|14|8|@]|D|@]|E|$]]]|U|$]]

<code>null == undefined</code> is true

<pre><code>if (arg == null){
 // arg was not passed.
}
</code></pre>

Code example:

<div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
<div class="snippet-code">
<pre class="snippet-code-js lang-js prettyprint-override"><code>var button = document.querySelector("button");

function myFunction(arg){
 if(arg == null){
 alert("argument was not passed.");
 } else {
 alert("argument " + arg + " was passed.");
 }
}</code></pre>
<pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;button onclick="myFunction('foo');"&gt;click to fire function w arg&lt;/button&gt;
&lt;br&gt;&lt;br&gt;
&lt;button onclick="myFunction();"&gt;click to fire function w/o arg&lt;/button&gt;</code></pre>
</div>
</div>

blocks|key|3044625|text|初始化默认值如果不存在：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3044626|function+loadDialog(fn,+f,+local,+anim)+{
++switch+(arguments.length)+{++
++++case+1:+f=null;
++++case+2:+local=false;
++++case+3:+anim=false;
++}
++...
}|code-block|syntax|javascript|3044627|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

init default values if not exists:
<pre><code>function loadDialog(fn, f, local, anim) {
 switch (arguments.length) { 
 case 1: f=null;
 case 2: local=false;
 case 3: anim=false;
 }
 ...
}
</code></pre>

blocks|key|3527955|text|这些答案对我来说都不起作用。我需要知道是否有人使用了这个参数。如果有人调用了传递undefined的函数，我希望将其作为参数使用该值来处理。无论如何，使用一些现代的JS解决方案非常简单：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3527956|function+parameterTest(...test)+{
++if+(test.length)+{
++++return+`exists`;
++}+else+{
++++return+`not+exists`;
++}
}

parameterTest(123);+++++++++//+exists
parameterTest(undefined);+++//+exists
parameterTest();++++++++++++//+not+exists
parameterTest(window.blah);+//+exists|code-block|syntax|javascript|3527957|对于较旧的浏览器，您可以使用参数：|3527958|function+parameterTest()+{
++if+(arguments.length)+{
++++return+"exists";
++}+else+{
++++return+"not+exists";
++}
}|3527959|entityMap^0|14|9|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|U|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|V|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|W|8|@]|D|@]|E|$]]]|P|$]]

None of these answers were doing it for me. I needed to know if someone had used the parameter or not. If someone invoked the function passing <code>undefined</code> I wanted to treat that as them using that value as the parameter. Anyway, the solution was quite easy using some modern JS:
<pre><code>function parameterTest(...test) {
 if (test.length) {
 return `exists`;
 } else {
 return `not exists`;
 }
}

parameterTest(123); // exists
parameterTest(undefined); // exists
parameterTest(); // not exists
parameterTest(window.blah); // exists
</code></pre>
For older browsers you can use arguments:
<pre><code>function parameterTest() {
 if (arguments.length) {
 return &quot;exists&quot;;
 } else {
 return &quot;not exists&quot;;
 }
}
</code></pre>

I was wondering, can you create a function with an optional parameter.

Example:

<pre><code>function parameterTest(test)
{
 if exists(test)
 {
 alert('the parameter exists...');
 }
 else
 {
 alert('The parameter doesn\'t exist...');
 }
}
</code></pre>

So if you call <code>parameterTest()</code> then the result would be a message "The parameter doesn't exist...". And if you call <code>parameterTest(true)</code> then it would return "the parameter exists...". 

Is this possible?

How to test if a parameter is provided to a function?

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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我想知道，你能创建一个带有可选参数的函数吗？示例：function parameterTest(test){   if exists(test)   {     alert('the parameter exists...');   }   else   {     alert('The parameter doesn...

问如何测试函数是否提供了参数？
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问如何测试函数是否提供了参数？EN