JavaScript分配变量以发出警报

问题描述

我想知道您可以分配一个变量来警报吗？它的真正含义和作用是什么？例如，

I am wondering Can you assign a variable to alert ? what does it really mean and do ? For example,

var a = alert('test');

我尝试了一下，并在页面加载到变量<$ c的位置时弹出警报当我调用它时，$ c> a 仍然为'undefined'。我们不是应该使它成为一个内部带有警报的匿名函数，例如

I tried it out, and the alert pops up as soon as the page loads where variable a remains 'undefined' when I call it. Aren't we suppose to make it an anonymous function with the alert inside like

var a = function(){ alert('test'); }

是否要触发某些警报？为什么Javascript允许您这样做？

If we want to trigger an alert on something? Why does javascript allow you to do that?

推荐答案

var a = alert('test');

请像其他变量赋值一样考虑该语句。为了执行分配，首先评估右侧。在这种情况下，这是一个函数调用，因此将调用该函数并获取其返回值。在警报的情况下，返回值为未定义。

Think of this statement like any other variable assignment. In order to perform the assignment, first the right-hand side is evaluated. In this case it's a function call, so the function is called and its return value is obtained. In alert's case, the return value is undefined. It doesn't return anything.

然后将返回值分配给 a ，因此 a 最终的值为 undefined 。

Then the return value is assigned to a, so a ends up with the value undefined.

语句，显示警告消息。发生这种情况的原因是调用alert（）并获取其返回值。

As a side effect of the statement, an alert message is displayed. That happens as a consequence of calling alert() and getting its return value.

function foo(x) {
    return x * 2;
}

var a = foo(3);

此代码在结构上与警报调用类似，会导致 a 为6。但是alert（）不会返回值。更像这样：

This code is structurally similar to the alert call, and would result in a being 6. But then alert() doesn't return a value. It's more like this:

function bar(x) {
    return;
}

var a = bar(3);

现在 a 是未定义。

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