Tilendor Tilendor - 1 year ago 196
Javascript Question

Set a default parameter value for a JavaScript function

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

Does this work in JavaScript?

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

Answer Source

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';
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