JS：函数的参数的默认值 [英] JS: functions arguments default values

问题描述

在某些语言可以为函数的参数设置默认值：

In some languages you can set default values for function's arguments:

function Foo(arg1 = 50, arg2 = 'default') {
    //...
}

你是怎么做到这在JavaScript中？

How do you do it in JavaScript?

推荐答案

在JavaScript中，任何未设置给定值未定义。这意味着，如果你想为一个功能设置默认值，你的第一线需要做个检查，看看如果这些值没有定义：

In JavaScript, anything that isn't set is given the value undefined. This means that if you want to set default values for a function, your first line needs to be a check to see if those values aren't defined:

function Foo(arg1, arg2) {
    if (typeof(arg1) === "undefined") { arg1 = 50; }
    if (typeof(arg2) === "undefined") { arg2 = "default"; }
}

如果你知道这些值将大致什么，你可以采取一些捷径的：

You can take a bit of a short cut if you know roughly what those values will be:

function Foo(arg1, arg2) {
    arg1 = arg1 || 50;
    arg2 = arg2 || "default";
}

但是，如果这些参数是falsy，他们将被替换。这意味着，如果ARG1是一个空字符串，空，未定义，假或 0 它会被改为50，所以要小心，如果你选择使用它。

However, if those arguments are falsy, they'll be replaced. This means that if arg1 is an empty string, null, undefined, false or 0 it'll be changed to 50, so be careful if you chose to use it

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