Set a default parameter value for a JavaScript function

First Solution

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';

Another Solution

To anyone interested in having there code work in Microsoft Edge, do not use defaults in function parameters.

function read_file(file, delete_after = false) {

In that example Edge will throw an error “Expecting ‘)’”

To get around this use

function read_file(file, delete_after) {
  if(delete_after == undefined)
    delete_after = false;

As of Aug 08 2016 this is still an issue

Reference link:


Leave a Reply

Your email address will not be published. Required fields are marked *

Enter Captcha Here : *

Reload Image