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|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|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|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|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|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>

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>

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。

文章

问答

视频

学习中心

腾讯云实验室

直播

竞赛

腾讯云代码分析专区

腾讯iOA零信任安全管理系统专区

腾讯云架构师技术同盟交流圈

腾讯云数据库专区

腾讯云顾问专区

腾讯云原生专区

腾讯混元专区

腾讯云TCE专区

腾讯云Lighthouse专区

腾讯云HAI专区

腾讯云Edgeone专区

腾讯云存储专区

腾讯云智能专区

腾讯轻联专区 

腾讯云开发专区

TAPD专区

腾讯轻量云游戏服专区

腾讯云最具价值专家

腾讯云架构师技术同盟

腾讯云创作之星

腾讯云开发者先锋

腾讯云代码助手

云原生构建

TAPD 敏捷项目管理

Cloud Studio

SDK中心

API中心

命令行工具

涵盖代码开发、场景应用、自动测试全流程，助你从零构建专属AI助手

一站式MCP教程库，解锁AI应用新玩法

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\'t exist...');   }}因此，如果调用parameterTest

问如何测试函数是否提供了参数？
EN

回答 9

Stack Overflow用户

Stack Overflow用户

Stack Overflow用户

社区

活动

圈层

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问如何测试函数是否提供了参数？EN

回答 9

Stack Overflow用户

Stack Overflow用户

Stack Overflow用户

社区

活动

圈层

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问如何测试函数是否提供了参数？
EN