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如何给JavaScript中函数的参数添加默认值.html
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如何给JavaScript中函数的参数添加默认值.html
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<title>如何给JavaScript中函数的参数添加默认值 | 李祖彬的博客</title>
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<meta name="description" content="前段时间在写js代码时,有个地方需要给函数的参数赋默认值.于是,我毫不犹豫地在参数后面直接给了值,如:
function sum(a,b=2) {}
*ES6已支持上面的语法
因为平时写PHP代码时都是这样,说实话以前还真没有给js参数赋过默认值,但是一写出来就报错了.不用说,肯定是js不能这样使.于是上网查找了 一番,终于明白在js里给参数默认值,可以这样:...">
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如何给JavaScript中函数的参数添加默认值
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<p>前段时间在写js代码时,有个地方需要给函数的参数赋默认值.于是,我毫不犹豫地在参数后面直接给了值,如:</p>
<pre class="line-numbers language-JavaScript" data-language="JavaScript"><code class="language-JavaScript">function sum(a,b=2) {}</code></pre>
<p><code>*ES6已支持上面的语法</code></p>
<p>因为平时写PHP代码时都是这样,说实话以前还真没有给js参数赋过默认值,但是一写出来就报错了.不用说,肯定是js不能这样使.于是上网查找了 一番,终于明白在js里给参数默认值,可以这样:</p>
<pre class="line-numbers language-JavaScript" data-language="JavaScript"><code class="language-JavaScript">function sum(a){
var b = arguments[1]?arguments[1]:2;
}</code></pre>
<p>马上写了个测试试试,还真可以.为什么可以这样写呢?原因是js在编译时会把参数放入arguments这个数组里面,这让我想起了PHP里的<code>func_get_args()</code>.但是,在一想,有问题啊!</p>
<p>是什么问题呢?js里面在作真假判断时,遇上<code>undefined</code>,<code>null</code>,<code>0</code>,<code>''</code>等这些情况时,是会返回假的,那么在这里,如果我调用这个函数,给b的值为零的话,那它不是会用默认值?情况真的是这样吗?试一试不就知道.</p>
<pre class="line-numbers language-JavaScript" data-language="JavaScript"><code class="language-JavaScript">function sum(a){
var b = arguments[1]?arguments[1]:2;
alert(a+b);
}
sum(1,0);</code></pre>
<p>结果,得到的还真是3,看来在使用的时候还需要注意一下啊.</p>
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