如何在 Javascript 中声明可选的函数参数？
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How can I declare optional function parameters in Javascript?
提问by Uttam Dutta
Can I declare default parameter like
我可以声明默认参数吗
function myFunc( a, b=0)
{
// b is my optional parameter
}
in javascript.
在 JavaScript 中。
回答by Tigraine
With ES6:This is now part of the language:
使用 ES6：这现在是语言的一部分：
function myFunc(a, b = 0) {
// function body
}
Please keep in mind that ES6 checks the values against undefinedand not against truthy-ness (so only real undefined values get the default value - falsy values like null will not default).
请记住，ES6 根据undefined而不是根据真实性检查值（因此只有真正的未定义值才会获得默认值 - 像 null 这样的虚假值不会默认）。
With ES5:
使用 ES5：
function myFunc(a,b) {
b = b || 0;
// b will be set either to b or to 0.
}
This works as long as all values you explicitly pass in are truthy.
Values that are not truthy as per MiniGod's comment: null, undefined, 0, false, ''
只要您明确传入的所有值都是true，这就会起作用。根据 MiniGod 的评论，不真实的值：null, undefined, 0, false, ''
It's pretty common to see JavaScript libraries to do a bunch of checks on optional inputs before the function actually starts.
看到 JavaScript 库在函数实际启动之前对可选输入进行大量检查是很常见的。
回答by Ja?ck
Update
更新
With ES6, this is possible in exactly the manner you have described; a detailed description can be found in the documentation.
使用 ES6，这完全可以按照您所描述的方式进行；详细说明可以在文档中找到。
Old answer
旧答案
Default parameters in JavaScript can be implemented in mainly two ways:
JavaScript 中的默认参数主要有两种实现方式：
function myfunc(a, b)
{
// use this if you specifically want to know if b was passed
if (b === undefined) {
// b was not passed
}
// use this if you know that a truthy value comparison will be enough
if (b) {
// b was passed and has truthy value
} else {
// b was not passed or has falsy value
}
// use this to set b to a default value (using truthy comparison)
b = b || "default value";
}
The expression b || "default value"evaluates the value AND existence of band returns the value of "default value"if beither doesn't exist or is falsy.
该表达式b || "default value"计算值 AND 的存在性b，"default value"如果b不存在或为假则返回值。
Alternative declaration:
替代声明：
function myfunc(a)
{
var b;
// use this to determine whether b was passed or not
if (arguments.length == 1) {
// b was not passed
} else {
b = arguments[1]; // take second argument
}
}
The special "array" argumentsis available inside the function; it contains all the arguments, starting from index 0to N - 1(where Nis the number of arguments passed).
特殊的“数组”arguments在函数内部可用；它包含所有参数，从索引开始0到N - 1（其中N是传递的参数数量）。
This is typically used to support an unknown number of optional parameters (of the same type); however, stating the expected arguments is preferred!
这通常用于支持未知数量的可选参数（相同类型）；但是，最好说明预期的参数！
Further considerations
进一步的考虑
Although undefinedis not writablesince ES5, some browsers are known to not enforce this. There are two alternatives you could use if you're worried about this:
虽然undefined是不可写，因为ES5，一些浏览器被称为不执行此。如果您对此感到担心，可以使用两种替代方法：
b === void 0;
typeof b === 'undefined'; // also works for undeclared variables
