javascript - 设置JavaScript函数的默认参数值(Set a default parameter value for a JavaScript function)

Question

I would like a JavaScript function to have optional arguments which I set a default on, which get used if the value isn't defined (and ignored if the value is passed).

(我希望JavaScript函数具有我设置了默认值的可选参数，如果未定义值，则使用该参数（如果传递值，则将其忽略）。)

In Ruby you can do it like this:

(在Ruby中，您可以这样操作：)

def read_file(file, delete_after = false)
  # code
end

Does this work in JavaScript?

(这可以在JavaScript中使用吗？)

function read_file(file, delete_after = false) {
  // Code
}

  ask by Tilendor translate from so

Answer 1

From ES6/ES2015 , default parameters are in the language specification.

(从ES6 / ES2015开始 ，默认参数位于语言规范中。)

function read_file(file, delete_after = false) {
  // Code
}

just works.

(正常工作。)

Reference: Default Parameters - MDN

(参考： 默认参数-MDN)

Default function parameters allow formal parameters to be initialized with default values if no value or undefined is passed.

(如果未传递任何值或未定义，则默认函数参数允许使用默认值初始化形式参数。)

You can also simulate default named parameters via destructuring :

(您还可以通过解构来模拟默认的命名参数 ：)

// the `= {}` below lets you call the function without any parameters
function myFor({ start = 5, end = 1, step = -1 } = {}) { // (A)
    // Use the variables `start`, `end` and `step` here
    ···
}

Pre ES2015 ,

(ES2015之前的版本)

There are a lot of ways, but this is my preferred method — it lets you pass in anything you want, including false or null.

(有很多方法，但这是我的首选方法-它使您可以传递任何想要的信息，包括false或null。)

( typeof null == "object" )

(（ typeof null == "object" ）)

function foo(a, b) {
  a = typeof a !== 'undefined' ? a : 42;
  b = typeof b !== 'undefined' ? b : 'default_b';
  ...
}