功能参数的默认值？ [英] Default value for function parameter?

问题描述

所以，只要我记得，我一直这样做，但我很好奇这是否真的应该做的事情。你编写一个带参数的函数，所以你预计它有一个值，但如果没有，你有充分的理由将它默认为零。我目前所做的是编写辅助函数：

So I've been doing this for as long as I can remember, but I'm curious if this is really what I should be doing. You write a function that takes a parameter, so you anticipate it to have a value, but if it doesn't, you have a good reason to default it, to say zero. What I currently do is write a helper function:

function foo() { return foo(0); };
function foo(bar) { ... };

我刚跑过一个我做过这个的实例，我奇怪地看了几秒钟之前理解我背后的逻辑。我来自php，这是微不足道的：

I just ran across an instance where I did this and I looked at it oddly for a few seconds before understanding my logic behind it. I come from php where it's trivial:

function foo(bar=0) { ... }

是否有我不知道的javascript替代方案？

Is there a javascript alternative that I'm not aware of?

推荐答案

您不能在JavaScript中使用重载函数。相反，使用基于对象的初始化，或检查值并指定默认值（如果没有提供）。

You can't have overloaded functions in JavaScript. Instead, use object based initialization, or check for the values and assign a default if none supplied.

在您的示例中，第二个函数 foo（ bar）将替换第一个。

In your example, the second function foo(bar) will replace the first one.

这是一个使用对象初始化的函数。

Here's a function using object initialization.

function foo(config) {
    extend(this, config);
}

其中 extend 是将配置对象与当前对象合并的函数。它类似于jQuery中的 $ .extend 方法，或者MooTools的 $ extend 方法。

where extend is a function that merges the config object with the current object. It is similar to the $.extend method in jQuery, or $extend method of MooTools.

调用函数并传递命名键值对

Invoke the function and pass it named key value pairs

foo({ bar: 0 });

初始化的另一种方法是查看提供的值，如果值是，则指定默认值没有给出

The other way to initialize is to look at the supplied values, and assign a default if the value is not given

function foo(bar) {
    bar = bar || 0;
}

只要bar不是假值，这就有效。所以 foo（false）或 foo（）仍将初始化 bar 到 0 。对于这种情况，请进行明确检查。

This works as long as bar is not a falsy value. So foo(false) or foo("") will still initialize bar to 0. For such cases, do an explicit check.

function foo(bar) {
    bar = (typeof bar == 'undefined' ? 0 : bar);
}

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