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 gets used if the value isn't defined. In Ruby you can do it like this:

def read_file(file, delete_after = false)

  # code

end

Does this work in JavaScript?

function read_file(file, delete_after = false) {

  // Code

}

Answer 1

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

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

just works.

Reference: Default Parameters - MDN

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

Pre ES2015,

There are a lot of ways, but this is my preferred method - it lets you pass in anything you want, including false or null. (typeof null == "object")

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